President Biden is slated to speak with Ukrainian President Volodymyr Zelensky on Thursday about the Russian forces gathering on the country's border.

The call with Zelensky will be followed by another during which Biden will brief the leaders of the Bucharest Nine (B9) about his two-hour phone call Tuesday with Russian President Vladimir Putin.

Ukrainian President Volodymyr Zelenskyy gestures as he addresses lawmakers in Kyiv, Ukraine, Wednesday, Dec.1, 2021. Ukraine marks the 30th anniversary of the referendum on independence. (AP Photo/Efrem Lukatsky)

Ukrainian President Volodymyr Zelenskyy gestures as he addresses lawmakers in Kyiv, Ukraine, Wednesday, Dec.1, 2021. Ukraine marks the 30th anniversary of the referendum on independence. (AP Photo/Efrem Lukatsky)

The B9 comprises the eastern flank of U.S.'s NATO allies, consisting of Bulgaria, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania, and Slovakia.

BIDEN SAYS SENDING US TROOPS TO UKRAINE ‘NOT ON THE TABLE’

Biden's phone call with Putin reportedly grew tense at times as they spoke about the roughly 100,000 troops massing along the Russia-Ukraine border.

"There were no minced words. It was polite, but I made it very clear: If in fact, he invades Ukraine, there'll be severe consequences," Biden told reporters Wednesday morning about the call.

President Joe Biden and Russian President Vladimir Putin

President Joe Biden and Russian President Vladimir Putin, arrive to meet at the 'Villa la Grange', Wednesday, June 16, 2021, in Geneva, Switzerland. (Saul Loeb/Pool via AP) (Saul Loeb/Pool via AP)

National Security Advisor Jake Sullivan announced the upcoming call between Biden and Zelensky on Tuesday.

"Our message to our friends in the Ukrainian government, as our message was today to President Putin, is that the United States supports the Minsk process, wants to see progress made towards a ceasefire, towards confidence-building measures. And that is the best way forward," Sullivan said.

US GOING TO WAR WITH RUSSIA OVER UKRAINE WOULD 'COURT DESTRUCTION OF THE KNOWN WORLD': MACGREGOR

Biden on Wednesday dismissed the possibility of sending U.S. troops to Ukraine amid the Russian military buildup on the country's border.

Russian President Vladimir Putin speaks during a news conference after his meeting with U.S President Joe Biden at the 'Villa la Grange' in Geneva, Switzerland in Geneva, Switzerland, Wednesday, June 16, 2021. (AP Photo/Alexander Zemlianichenko, Pool)

Russian President Vladimir Putin speaks during a news conference after his meeting with U.S President Joe Biden at the 'Villa la Grange' in Geneva, Switzerland in Geneva, Switzerland, Wednesday, June 16, 2021. (AP Photo/Alexander Zemlianichenko, Pool)

"That is not on the table," Biden told reporters regarding whether the U.S. would send troops to Ukraine. "We have a moral obligation and a legal obligation to our NATO allies if they were to attack under Article 5, that's a sacred obligation. That obligation does not extend to ... Ukraine."

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Fox News' Michael Lee contributed to this report.