President Joe Biden called on Russian President Vladimir Putin on Tuesday to “de-escalate tensions” following a Russian military buildup on Ukraine’s border.

Biden also told Putin the U.S. would “act firmly in defense of its national interests” regarding Russian cyber intrusions and election interference, according to the White House. Biden proposed a summit meeting in a third country “in the coming months” to discuss the full range of U.S.-Russia issues, the White House said.

The call comes as concern rises in the West about a surge of cease-fire violations in eastern Ukraine, where Russia-baсked separatists and Ukrainian forces have been locked in a conflict since Moscow’s 2014 annexation of Ukraine’s Crimean Peninsula.

“President Biden emphasized the United States’ unwavering commitment to Ukraine’s sovereignty and territorial integrity,” the White House said in a statement. The White House added that Biden made clear that Russia must “de-escalate tensions.”

More than 14,000 people have died in fighting in eastern Ukraine, and efforts to negotiate a political settlement have stalled. Over the past week, there have been daily reports of Ukrainian military casualties and rebels also have reported losses.

Ukraine has said Russia has massed 41,000 troops at its border with eastern Ukraine and 42,000 more in Crimea. Russia’s defense minister said Tuesday the buildup was part of readiness drills in response to what he described as threats from NATO, and he said the maneuvers in western Russia would last for two more weeks.

The Kremlin in a statement said “during an exchange of opinions on Ukraine’s internal political crisis,” Putin told Biden about “approaches to a political settlement” based on the 2015 peace deal brokered by France and Germany and signed in Minsk, Belarus.

The two also discussed arms control, the Iranian nuclear program, the situation in Afghanistan and climate change, according to the Kremlin.

Earlier Tuesday, Secretary of State Antony Blinken, who was in Brussels for meetings with NATO allies, accused Russia of taking “very provocative” actions with the amassing of troops.

NATO chief Jens Stoltenberg also called the Russian movements “unjustified, unexplained and deeply concerning.”

Ukrainian Foreign Minister Dmytro Kuleba said Kyiv welcomed the support and urged Western countries to make clear to Moscow that it would pay a price for its “aggression.”

The Biden-Putin relationship has been rocky in the early going of the new U.S. administration. Although Biden agreed to extend a major arms control deal with Russia, he has been notably cool toward Moscow and highly critical of many of its activities.

Last month in an interview with ABC News, Biden agreed with the description of Putin as a “killer” and he has criticized the jailing of Russian opposition figure Alexei Navalny. U.S. intelligence released a report last month finding that Putin authorized influence operations to help former President Donald Trump’s reelection bid.

Putin in response to Biden’s killer comment recalled his ambassador to the U.S. and pointed at the U.S. history of slavery and slaughtering Native Americans and the atomic bombing of Japan in World War II.

The back and forth underscored Biden’s desire to distance himself from the perception that Trump was soft on Putin despite actions his administration took against Russia.

In his first call with Putin as president in late January, Biden raised concerns about the arrest of Navalny, Russia’s alleged involvement in a vast cyberespionage campaign and reports of Russian bounties on American troops in Afghanistan. The Kremlin, meanwhile, focused on Putin’s response to Biden’s proposal to extend the last remaining U.S.-Russia arms control treaty.

Days after that call, Biden said in a speech before State Department officials that the days of the U.S. “rolling over” to Putin were over.

“We will not hesitate to raise the cost on Russia and defend our vital interests and our people,” Biden said.

(AP)