The White House stated that President Joe Biden called Volodymyr Zelenskyy, Ukrainian President, by telephone on Sunday to reaffirm U.S. support, as the country faces increasing Russian aggression.

Western and the U.S. allies have watched Russia build up 100,000 or so troops along the border with Ukraine, prompting fears of an invasion as early as this month. Russia denies that any plan is being made to attack the neighboring country, however officials of the Biden administration said they were ready for it.

Sunday’s call with Zelenskyy — the second the two leaders have held in recent weeks — came days after Biden urged Russian President Vladimir Putin to de-escalate tensions on Ukraine’s border.

Biden informed Zelenskyy the U.S., its allies and Partners will respond decisively if Russia invades Ukraine again. Jen Psaki, White House Press Secretary, stated this in a statement following the conclusion of the conference call.

Psaki stated that Biden had emphasized the United States’ commitment to its allies, partners and the principle of “nothing but you” without referring specifically to Biden. He also supported confidence-building measures that de-escalate tensions within Donbas, and active diplomacy in order to promote the implementation of Minsk Agreements. This was in support of Normandy Format.

In a tweet, Zelenskyy praised the “unwavering support” that he said Ukraine has received from the U.S.

Recent weeks have seen high stakes diplomatic negotiations between Russia and America over Ukraine.

Biden was just days away from Sunday’s phone call with Zelenskyy. urged Russian President Vladimir Putin to de-escalate tensions on Ukraine’s border. during a 50-minute phone call on Thursday, Biden warned Putin that the U.S. could impose new sanctions against the country if it takes military action against Ukraine. Russians requested their call. It was Biden’s second last month between Putin and Biden.

“We have made it very clear to President Putin, that any further moves or intrusions into Ukraine will result in severe sanctions. Biden said that the United States will expand its presence in Europe, with NATO allies, and there will be severe sanctions.

Biden’s comments were very similar to the remarks he made in an earlier call with Putin, where he said that Moscow will face “severe penalties” for any actions it takes on Ukraine.

TASS Russian news agency reported that Putin called the reporters to warn them of the possibility of a severe break in relations between their two countries if Biden tries imposing a series sanctions.

Ned Price, a spokesperson from the State Department said that Antony Blinken called Zelenskyy Wednesday to reiterate U.S. support in Ukraine’s independence, sovereignty, and territorial integrity despite Russia’s militarism at the border.

Price reported that they had discussed the possibility of peacefully resolving conflict in Ukraine’s eastern region and future diplomatic engagements with Russia.

Security talks between the U.S., Russia and NATO officials have been scheduled for Geneva, Switzerland on January 10. Biden, Putin and others are unlikely to be there. An international meeting of NATO and Russia is planned for January 12.