WASHINGTON xe2x80x94 For the Russian military, the difficult part came quickly.

On the first day of President Vladimir V. Putinxe2x80x99s invasion of Ukraine, his generals and troops followed a textbook strategy for land invasions. They attacked the countryxe2x80x99s military installations and air defense systems with missiles launched from the air, sea and land, seeking to take ownership of the skies, and sped forces to Kyiv, the capital, with the goal of decapitating the government of the democratically elected president, Volodymyr Zelensky.

But then, things slowed. It is one thing to cross the border of another country with tanks and artillery, protected by warplanes above, Pentagon officials and analysts say. It is another thing entirely to lay siege to cities and an army populated by people willing to put their lives on the line to protect what they view as their sovereign right to self-determination.

Within a day of entering Ukraine, Russian forces lost some momentum, senior American and British officials said, as Ukrainian fighters mounted a resistance. No population centers had been taken, a senior Defense Department official told reporters at the Pentagon on Friday. Nor had Russia yet managed to achieve air superiority over Ukraine, partly because the Ukrainians are using mobile systems and partly because Russian missiles have hit old air defense sites, which could show a flaw in Russiaxe2x80x99s intelligence. The Ukrainian air defense and missile defense systems were degraded, he said, but the countryxe2x80x99s air force was still flying planes and denying air access to Russia.

In addition, officials said, Russia was conducting most of its initial operations during the day, suggesting that its ability to fight at night xe2x80x94 a hallmark of the American military xe2x80x94 was less effective.

xe2x80x9cUkrainian armed forces continue to offer strong resistance,xe2x80x9d said Lt. Gen. Jim Hockenhull, the British chief of defense intelligence.

That said, Pentagon officials warned that Russia had sent in only one-third of the 150,000 to 190,000 troops it had massed at Ukrainexe2x80x99s borders, so Moscow could intensify pressure at any time. Military officials said Russia was still in the initial phases of an operation that could take two to three weeks to seize most of the country.

Officials said Russia had begun an amphibious assault from the Sea of Azov, near Mariupol, in the south. Thousands of Russian naval infantry are coming ashore there, with military officials assessing that the plan is to move toward the city. The Russian military, with its decisive edge in cyberwarfare, tanks, heavy weaponry, missiles, fighter planes, warships and sheer numbers, dwarfs that of Ukraine.

But wars are not fought on paper alone. While Russia has established attack lines into three cities xe2x80x94 Kyiv in the north, Kharkiv in the northeast and Kherson in the south xe2x80x94 Ukrainian troops are fighting to hold all three. Significantly, the senior U.S. defense official said, Ukrainian command and control remains intact.

Russiaxe2x80x99s attack lines are bottlenecked, a second official said, as Ukrainian troops fiercely engage against the Russians. The resistance, the official said, is why the Russian troops massed at the border have not all crossed. But the official warned that more of those troops would flow quickly to the cities xe2x80x94 particularly Kyiv xe2x80x94 if the forward elements break the Ukrainian troops who have held them up.

xe2x80x9cItxe2x80x99s not apparent to us that Russian forces over the past 24 hours have been able to execute their plans as they deemed they would,xe2x80x9d John F. Kirby, the Pentagonxe2x80x99s chief spokesman, said later Friday. xe2x80x9cBut itxe2x80x99s a dynamic situation.xe2x80x9d

As some Russian troops entered a northern district of Kyiv, missile strikes hammered the city and rockets crashed into residential buildings. If Russian intelligence has figured out where Mr. Zelensky and the rest of the Ukrainian leadership are hiding, the Russian military will probably try to take them out with rockets and airstrikes, a senior Biden administration official said in an interview. But if that does not work, Russian forces might resort to urban combat, a more difficult endeavor.

xe2x80x9cThe easy part is attacking with missiles and hitting airfields,xe2x80x9d said retired Col. David Lapan, a 30-year veteran of the Marine Corps. xe2x80x9cBut the narrative that theyxe2x80x99ve overrun Ukraine is very premature. Wexe2x80x99re just a couple of days into this, and it could go on a long time.xe2x80x9d

Senior Pentagon officials echoed that view. Russian troops are surrounding Kyiv with an aim to isolate and possibly lay siege to the capital, the senior Biden official said. He said the Russian forces had a list of Mr. Zelenskyxe2x80x99s leadership team, and would seek to kill or capture those officials if targeted airstrikes did not accomplish Mr. Putinxe2x80x99s aim of eliminating the government. But Ukrainian troops and citizens are fighting back, he said, which means that Russia, for all of its military might, may not have an easy time reaching its objectives. It would get bloody, he said.

It already has. In an exchange captured in an audio recording that has been shared and tweeted around the world, a Russian warship ordered 13 soldiers protecting tiny Snake Island in the countryxe2x80x99s south to xe2x80x9csurrenderxe2x80x9d or xe2x80x9cbe bombed.xe2x80x9d A Ukrainian border guard defiantly responded: xe2x80x9cRussian warship,xe2x80x9d then used an expletive to reject the demand.

The warship opened fire and killed all 13 border guards. That small battlefield victory for Russia, one Pentagon official said, could inspire Ukrainians and cost Mr. Putin in the public eye at home.

xe2x80x9cThe Ukrainians are badly overmatched in technology and sheer combat power, especially in the air and at sea, but are fighting on their homeland to protect their children and families,xe2x80x9d said retired Adm. James G. Stavridis, the former supreme allied commander for Europe. xe2x80x9cMotivation is far higher on their side, and the intangibles can help.xe2x80x9d

The Russian military attack continued on Friday as it started the day before: with the terrifying thud of artillery strikes on airports and military installations all over Ukraine.

The Pentagon said that the Russians, using missiles and long-range artillery, were facing particularly strong resistance near Kyiv and Kharkiv.

But Russian troops were also ratcheting up their cyberattacks on media sites and other communications, as well as against a major dam that supplies power across southern Ukraine, the senior Pentagon official said. He added, however, that Mr. Zelensky and his top civilian aides were still in communication with Ukrainian commanders.

Why Russia has not launched even larger cyberattacks across the country, and shut down virtually all communications, to cut off military units from their commanders in Kyiv and from each other remained a bit of a mystery on Friday.

U.S. military officials said it could be that efforts to safeguard Ukrainexe2x80x99s communications in anticipation of a major Russia attack were helping. Or, since it is believed that many of Ukrainexe2x80x99s internet and phone communications go through Russia, Moscow might be leaving some lines open to eavesdrop on Ukrainian civilian and military officials.

By Thursday night, Russian special forces and airborne troops had pushed into the outskirts of Kyiv. And on Friday, Russian airborne forces had blocked Kyiv from the west, the Defense Ministry claimed, after capturing an airfield in the area in an assault that used xe2x80x9cmore than 200 Russian helicopters.xe2x80x9d If accurate, that could create an air bridge that allows Russia to fly in hundreds of troops to help encircle the capital.

Ukrainian forces, which officials said had shot down several Russian jets and a helicopter in the earlier hours of the conflict on Thursday, were battling all along a broad front line to maintain control over their country.

By midday Friday, Russian forces had fired more than 200 missiles, mostly short-range ballistic rockets but also cruise missiles and rockets fired from the Black Sea, at targets across Ukraine, according to the senior Pentagon official, who spoke on the condition of anonymity to discuss sensitive military assessments.

The targets were primarily military: barracks, ammunition depots and air fields, the official said, in an expected move to destroy as much of the outgunned Ukrainian military as possible, as well as to help weaken any guerrilla movement that could rise up from the ashes of a defeated Ukrainian army.

Russia insisted it was not bombing civilian targets and was trying to limit casualties in the Ukrainian military. xe2x80x9cNo strikes against civilian infrastructure are being carried out,xe2x80x9d Foreign Minister Sergey V. Lavrov said Friday.

But the senior Pentagon official challenged that claim, saying some civilian residential areas had been struck, though the official could not say if they had been deliberately targeted.

How the battle for Kyiv ends will likely signal Mr. Putinxe2x80x99s larger plans for Ukraine.

xe2x80x9cPutinxe2x80x99s M.O. is to install new government and have them do the dirty work,xe2x80x9d said Representative Michael Waltz, a Florida Republican and former Army Green Beret who traveled to Ukraine in December. xe2x80x9cItxe2x80x99s unclear if he is underestimating the level of Ukrainian nationalism thatxe2x80x99s developed since 2014.xe2x80x9d

Mr. Waltz said he met with Ukrainexe2x80x99s top commando on that December trip. xe2x80x9cHe was very focused on developing a resistance organization,xe2x80x9d Mr. Waltz said, xe2x80x9cbut Ixe2x80x99m not sure he had enough time.xe2x80x9d