KYIV, Ukraine – U.S. President Joe Biden warned Thursday that Russia could still invade Ukraine within days, and the No. 2 diplomat at the U.S. Embassy in Moscow was expelled as tensions flared anew in the worst East-West standoff in decades.

NATO allies accused Russia of misleading the world with “disinformation” by saying it was returning some troops to their bases xe2x80x93 one of the gestures Russia made this week that briefly cooled temperatures and raised hopes for peace. Russia is believed to have some 150,000 forces around Ukraine’s borders.

Speaking at the White House, Biden said Washington saw no signs of a Russian withdrawal of forces, and said the U.S. has “reason to believe” that Russia is “engaged in a false flag operation to have an excuse to go in.”

He told reporters: “Every indication we have is theyxe2x80x99re prepared to go into Ukraine, attack Ukraine.”

The State Department said Russia ordered the deputy chief of mission to the U.S. Embassy in Moscow, Bart Gorman, to leave the country, calling the move “unprovoked” and “an escalatory step.” Russia provided no details of why he was expelled.

Tensions also spiked along the line that separates Ukrainian forces from Russia-backed separatists in the countryxe2x80x99s east, with the parties accusing each other of intensive shelling.

Russia held out an offer of diplomacy, handing the U.S. a response Thursday to offers to engage in talks on limiting missile deployments in Europe, restrictions on military drills and other confidence-building measures.

The response, published by the Foreign Ministry, deplored the West’s refusal to meet the main Russian security and demands and reaffirmed that Moscow could take unspecified “military-technical measures” if the U.S. and its allies continue to stonewall its concerns.

At the same time, it said Russia was ready to discuss measures to enhance security in Europe by discussing limits on missile deployments, restrictions on patrol flights by strategic bombers and other confidence-building steps.

U.S. Secretary of State Antony Blinken headed to New York for the U.N. Security Council meeting and then Germany for the Munich Security Conference.

Western powers estimate Russia has 150,000-plus troops massed outside Ukraine’s borders.

“Wexe2x80x99ve seen some of those troops inch closer to that border. We see them fly in more combat and support aircraft,” U.S. Defense Secretary Lloyd Austin said at NATO headquarters in Brussels. “We see them sharpen their readiness in the Black Sea. We even see them stocking up their blood supplies. You donxe2x80x99t do these sort of things for no reason, and you certainly donxe2x80x99t do them if youxe2x80x99re getting ready to pack up and go home.”

British Defense Secretary Ben Wallace said the West has seen “an increase of troops over the last 48 hours, up to 7,000.” That squared with what a U.S. administration official said a day earlier. The top EU official said similar.

British Armed Forces Minister James Heappey even called Russiaxe2x80x99s claim to be withdrawing troops “disinformation.” Russia accuses the West of the same.

Russia has “enough troops, enough capabilities, to launch a full-fledged invasion of Ukraine with very little or no warning time,” NATO Secretary-General Jens Stoltenberg said. “The fact that youxe2x80x99re putting a battle tank on a train and moving it in some direction doesnxe2x80x99t prove a withdrawal of troops.”

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Moscow said several times this week that some forces are pulling back to their bases, but it gave few details that would allow for an independent assessment of the scope and direction of the troop movement.

Russian Defense Ministry spokesman Maj.-Gen. Igor Konashenkov offered a bit more detail Thursday, saying that Russian tank and infantry units that took part in drills in the Kursk and Bryansk regions neighboring Ukraine were pulling back to their permanent bases in Nizhny Novgorod region. He said that some of those units already had arrived at their bases after a 700-kilometer journey east.

Troops deployed for exercises in Crimea, which Russia annexed from Ukraine in 2014, have moved back to Chechnya and Dagestan in Russia’s North Caucasus, he noted. He also said Russian troops involved in drills Belarus will also move back to their garrisons after war games there wrap up on Sunday. Konashenkov didn’t mention the numbers of troops that were deployed and didn’t say how many of them returned.

NATO, meanwhile, has moved troops and military equipment into Eastern Europe in a display of resolve of meant to deter any Russian aggression and underline its intent to defend NATOxe2x80x99s eastern members, in the unlikely event that they too become a target.

The U.S. has started deploying 5,000 troops to Poland and Romania. Another 8,500 are on standby, and some U.S. troops are expected to move toward Bulgaria. Britain is sending hundreds of soldiers to Poland, offering more warships and planes, and doubling its personnel in Estonia. Germany, the Netherlands and Norway are sending additional troops to Lithuania. Denmark and Spain are providing jets for air policing in the Baltic Sea region.

Even if an attack doesn’t materialize, the sustained Russian pressure on Ukraine has further hobbled its shaky economy and left an entire nation under constant strain xe2x80x94 a situation that could last indefinitely.

Ukraine already has been the stage of fighting for eight years, and tensions soared again Thursday in the conflict in the countryxe2x80x99s east, where Russian-backed separatists have been fighting Ukrainian troops since 2014.

Separatist authorities in the Luhansk region reported an increase in Ukrainian shelling along the tense line of contact, describing it as a “large-scale provocation.” Separatist official Rodion Miroshnik said rebel forces returned fire.

Ukraine disputed the claim, saying that separatists had shelled its forces, but they didnxe2x80x99t fire back. The Ukrainian military command charged that shells hit a kindergarten building in Stanytsia Luhanska, wounding two civilians, and cut power supply to half of the town.

An observer mission of the Organization for Security and Cooperation in Europe is expected to offer its assessment of the situation later Thursday.

Ukrainain President Volodymyr Zelenskyy tweeted that “the shelling of a kindergarten in Stanytsia Luhanska by pro-Russian forces is a big provocation,” adding that the OSCE monitoring activities are “an additional deterrent.”

Asked about the flare-up of hostilities in the east, Stoltenberg said the alliance was concerned “that Russia is trying to stage a pretext for an armed attack against Ukraine.”

Russia, in turn, aired worries that hawkish forces in Ukraine, encouraged by the West, could launch an attack to reclaim control of the rebel areas xe2x80x94 plans Ukrainian authorities deny.

A 2015 deal brokered by France and Germany helped end the worst of the fighting in eastern Ukraine, but regular skirmishes have continued and a political settlement has stalled.

The U.N. Security Council is scheduled to hold its annual meeting on the agreement on Thursday.

Russia denies it is plotting an invasion but says it’s free to deploy troops wherever necessary to counter NATO threats. It wants the West to keep Ukraine and other former Soviet nations out of NATO, halt weapons deployments near Russian borders and roll back forces from Eastern Europe, the demands the allies have flatly rejected.

While the U.S. and its allies have rejected Moscow’s demands to bar membership to Ukraine, they offered to engage in talks with Russia on limiting missile deployments in Europe, restrictions on military drills and other confidence-building measures.

Russian President Vladimir Putin charged that Moscow had offered to discuss those issues years ago, but the West only agreed to talk about them now. He said that Russia was ready to talk about them now, but only in conjunction with its main security demands.

Even as Russia seemed to try to ease tensions this week, Maxar Technologies, a commercial satellite imagery company that has been monitoring the Russian buildup, reported continued heightened military activity near Ukraine. It noted a new pontoon bridge and a new field hospital in Belarus. It also said that some forces had left an airfield in the country, a Russian ally, but it was unclear where they went.

Blinken and U.S. Vice President Kamala Harris were among political, military and diplomatic leaders heading to the annual security conference in Munich that will see urgent consultations on the crisis.

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Isachenkov reported from Moscow, Superville from Washington and Cook from Brussels. Matthew Lee in Munich, Angela Charlton in Paris, Jill Lawless in London, Edith M. Lederer at the United Nations, Frank Jordans in Berlin, and Aamer Madhani, Ellen Knickmeyer, Colleen Long and Zeke Miller in Washington contributed.