• President Joe Biden plans to speak with Ukraine President Zelenskyy on Sunday to express support amid Russia's military buildup on the border, a White House official said.
• The U.S. intends to review plans for diplomatic moves aimed "to help de-escalate the situation," the official said.
• Biden this week urged Russian President Putin to lower tensions with Ukraine, and warned that the U.S. was prepared to "respond decisively" if Russia invades.

President Joe Biden plans to speak with Ukraine President Volodymyr Zelenskyy on Sunday to express support amid Russia's military buildup near the countries' shared border, a White House official said Friday.

Biden also intends to review plans for diplomatic moves aimed "to help de-escalate the situation in the region," the official said.

"I'm not going to negotiate here in public," Biden told reporters in Delaware on Friday afternoon. "But we made it clear [Russian President Vladimir Putin] cannot, I'll emphasize, cannot invade Ukraine."

U.S. and Russian officials are set to hold security talks on Jan. 10 to discuss the Russia-Ukraine tensions and other issues.

On Thursday, Biden in a 50-minute phone call urged Putin to lower those tensions. He warned the U.S. was prepared to "respond decisively if Russia further invades Ukraine," White House press secretary Jen Psaki said.

"I made it clear to President Putin that we will have severe sanctions, we will increase our presence in Europe, with NATO allies," Biden said on Friday.

Asked if he sensed that Putin was less likely to invade Ukraine after their talk, Biden said, "Well, [what] I got the sense of is that he's agreed that we would have three major conferences in Europe" next month.

"He laid out some of his concerns about NATO and the United States and Europe, we laid out ours," Biden said. "I made it clear that they only could work if he de-escalated."

"I always expect to negotiate and make progress," Biden added.

A Kremlin aide reportedly said after the phone call with Biden that Putin "outlined in detail the basic principles laid down by Russia in the security proposals and emphasized that we will seek to ensure Russia's security."

Ukraine for months has sounded alarms about the buildup of thousands of troops along its eastern border with Russia. Putin has previously insisted that, despite the military deployment, Moscow is not preparing to invade. Pro-Russian separatists, meanwhile, have pushed into swaths of eastern Ukraine.

Putin has opposed the prospect of Ukraine joining NATO, the 30-nation intercontinental military alliance where an attack on one member is considered an attack on all. Ukraine has attempted to join NATO for years.

In late November, Putin suggested Russia would retaliate if Western governments supplied Ukraine with certain weapons systems. He has called the expansion of NATO's military power into other eastern European nations a threat.

Russia in 2014 annexed the Crimean Peninsula from Ukraine. Putin has argued that Ukraine is essentially a part of Russia.

Biden held separate calls earlier in December with both Putin and Zelenskyy. Biden and Zelenskyy met at the White House in September.