President Joe Biden spoke with Russian President Vladimir Putin for the second time this month Thursday. According to White House officials, the call began at 3:35 p.m. EST and lasted for about 50 minutes.

Like the call earlier this month, the situation in Ukraine dominated the conversation. Since the first meeting, Russia has moved an estimated 100,000 troops toward Ukraine. A report saying Russia would be pulling back about 10,000 of those troops came out over the weekend.

“We are prepared to provide Ukraine with further assistance to defend its territory and respond to a potential Russian occupation should a further invasion proceed in the coming weeks,” a senior Biden Administration official said during a background call previewing Thursday’s Biden-Putin talks. During the call, President Biden warned President Putin that the United States could impose new sanctions against Russia if it takes further military action against Ukraine.

According to White House Press Secretary Jen Psaki, Biden “urged Russia to de-escalate tensions with Ukraine” and “made clear that the United States and its allies and partners will respond decisively if Russia further invades Ukraine.” It remains unclear what, if anything, Biden would be willing to offer Putin in exchange for deescalating the situation.

In response, Putin said new sanctions could lead to a complete rupture of ties between the nations. After the call Putin’s foreign affairs adviser Yuri Ushakov said imposing sanctions “would be a colossal mistake that would entail grave consequences.”

“A lot of mistakes have been made over the past 30 years, and we would better avoid more such mistakes in this situation,” Ushakov said.

Thursday’s call also came just under two weeks after Russia published a draft of its security demands to the United States and NATO. Further talks on those demands are set for next month.

“Russia has put its concerns on the table and we are prepared to discuss them,” the official said. “The United States and our allies and partners will put our concerns on the table and expect Russia to be prepared to discuss them as well.”

Experts expected Biden and Putin to discuss a range of additional topics, including the slow-moving negotiations with Iran over its nuclear program.