WASHINGTON xe2x80x93xc2xa0President Joe Bidenxc2xa0spoke with Russian President Vladimir Putin for nearly an hour Thursday amid growing concerns over Moscow’s military buildup along the border with Ukraine.xc2xa0

Biden told Putin that the United States and its allies “will respond decisively if Russia further invades Ukraine,xe2x80x9d White House press secretary Jen Psaki said in a statement released after the phone call.xc2xa0

A senior administration official said Biden told Putin he has two options: diplomacy leading to de-escalation or further aggression leading to xe2x80x9cserious costs and consequences.xe2x80x9d

The possible consequences include economic sanctions, adjustments of NATO forces in allied nations and additional assistance to Ukraine to defend its territory, said the official, who briefed reporters on the condition of anonymity.

The White House has repeatedlyxc2xa0warned Russia against invading its neighborxc2xa0xe2x80x94 with a senior administration official labeling the situation a “moment of crisis” Wednesdayxc2xa0xe2x80x94 and Bidenxe2x80x99s call with Putin marked a new test of American diplomacy and leadership.xc2xa0

xe2x80x9cBiden is under pressure to show results and to show whether the United States, with its allies, can deter Russia and get it to step down from this build up,xe2x80x9d said Angela Stent, director of the Center for Eurasian, Russian and East European Studies at Georgetown University.

Biden’s call with Putin lasted roughly 50 minutes, ending at 4:25 p.m. EST, according to the White House.

Russia has ramped up its military presencexc2xa0along Ukraine’s eastern border over the past several months. Biden and America’s Westernxc2xa0allies have said they will impose tough economic sanctions against Russia if its aggression against Ukraine escalates.

Ukraine declaredxc2xa0independencexc2xa0from the former Soviet Union in 1991, and it is now a fledgling democracy with a pro-Western tilt.xc2xa0U.S. foreign policy experts say it is in Americaxe2x80x99s interest to stave off Russian aggression, particularly as Putin looks to rebuild the Soviet empire.

Russia annexed thexc2xa0Crimean peninsulaxc2xa0in 2014, a swath of Ukrainianxc2xa0territory located between the Black Sea and the Sea of Azov. The Kremlin has continued to try to undermine the countryxe2x80x99s sovereignty, militarily and in other ways.

The White House has previously said discussions with Russia would be more productive if Putin withdraws some of thexc2xa0estimated 100,000 Russian troops from Ukraine’s border.xc2xa0However, troop presence has remained consistent, worrying the White House.

What is happening at Ukraine’s border?:Putin’s buildup of Russian troops sparks concern

Biden and Putinxe2x80x99s call camexc2xa0ahead of a bilateral meeting between U.S. and Russian officials scheduled for Jan. 10.xc2xa0That same week, there will also be meetings between NATO and Russia, as well as the Organization for Security and Cooperation in Europe, where Ukraine will be present.

The administration official said Biden viewed his conversation with Putin on Thursday as an opportunity to lay the groundwork for the upcoming January meetings.xc2xa0

xe2x80x9cBoth leaders acknowledged that there were likely to be areas where we could make meaningful progress,xe2x80x9d the official said.

Putin has said he wantsxc2xa0the U.S. and Western allies to not allow Ukraine to join NATO xe2x80x94 a move he argues would pose a threat to Russian security. The Russian leader has also demanded that NATO no longer station troops orxc2xa0military equipment in countries that were formerly part of thexc2xa0Soviet Union.

xe2x80x9cWe have made it clear that any further movement of NATO to the East is unacceptable,xe2x80x9d Putin said at a news conference on Dec. 23. xe2x80x9cIs there anything unclear about this? Are we deploying missiles near the US border? No, we are not. It is the United States that has come to our home with its missiles and is already standing at our doorstep.”xc2xa0

Biden held a high-stakes video teleconference with Putin earlier this month, where he emphasized that he preferred a diplomatic solution to the crisis in Ukraine. However, the president warned that if Russia invades Ukraine, the U.S. would send additional defense resources to Ukraine and would look to deploy additional forces to fortify its NATO allies in the area.

Contributing: Michael Collins, Matthew Brown and Joey Garrison

Reach Rebecca Morin at Twitter @RebeccaMorin_