WASHINGTON (AP) — President Joe Biden reached out to Ukraine's leader Thursday as the United States moved to take a more direct role in diplomacy between that country and Russia, part of a broader effort to dissuade Russia from a destabilizing invasion of its western neighbor.

But any negotiations to peacefully resolve Europe's tangled East-West rivalries will present minefields for the U.S. president.

Biden made his offer of American diplomacy during a two-hour online session with Russian President Vladimir Putin on Tuesday. Biden proposed joining the Europeans in negotiations not just to settle the conflict in eastern Ukraine but to address Putin's larger strategic objections to NATO expanding its membership and building military capacity ever closer to Russia's borders.

Biden spoke with Ukrainian President Volodymyr Zelenskyy by phone for more than an hour Thursday. Biden assured Zelenskyy of support for Ukraine's sovereignty and territorial integrity. He also renewed a pledge that the U.S. and its allies would hit Russia with economic sanctions and intensify defensive aid to Ukraine and NATO allies nearest to Russia if Russia did invade, the White House said.

The two called on Putin to calm the crisis and urged diplomacy. Biden said the U.S. was prepared to help with "confidence-building measures" to implement a 2015 peace deal.

In a statement after the call with Biden, Ukraine said Zelenskyy was offering "clear proposals to unblock the peace process and is ready to discuss them in various formats."

Administration officials have suggested that the U.S. will press Ukraine to formally cede a measure of autonomy within its eastern Donbas region, which is now under de facto control by Russia-backed separatists who rose up against Kyiv in 2014.

Decentralization of Ukraine and a "special status" for Donbas were laid out in an ambiguous, European-brokered peace deal in 2015, but it has never taken hold.

More autonomy could formally give residents of that region more authority over some local issues. The administration officials made no mention of ceding any territory.

Biden also will have to finesse Ukraine's desire to join NATO. The U.S. and NATO reject Putin's demands that they guarantee Ukraine won't be admitted to the Western military alliance.

But senior State Department officials have told Ukraine that NATO membership is unlikely to be approved in the next decade, according to a person familiar with those private talks who spoke on condition of anonymity.

For Biden, the challenge will be encouraging Kyiv to accept some of the facts on the ground in eastern Ukraine, without appearing to cave to Putin — a perception that could embolden the Russian leader and unleash a fresh line of condemnations by Republicans as Biden's popularity is already in decline.

Ukraine may be asked "can you make some step forward on these areas," said Steven Pifer, a former U.S. ambassador to Ukraine. That could include measures such as allowing the Russia-allied Donbas region to control its own health care, police and schools, he said.

"But I don't see Washington pushing the Ukrainians to take steps that would compromise their sovereignty or the ability of the national government when it came to making decisions," Pifer said.

Ukraine, a former Soviet republic with deep cultural and historic ties to Russia, has in recent years sought closer integration with the West and membership in NATO. The alliance has held out the promise of membership but it has declined to set a timeline. Even before the current crisis, Ukraine was a long way from joining.

Since 2014, however, when Russia invaded and annexed Crimea and then threw its weight behind the armed separatists in the industrial Donbas region, the United States and other NATO members have been helping Ukraine build up its defenses.

The call between Biden and Putin took place as tensions grew over the threat of Russian forces again rolling into Ukraine. Putin denies any such intention and charges that it is NATO strengthening its hold in former Soviet satellites and republics that is threatening Russia.

U.S. intelligence reports last week said Russia had moved 70,000 troops to Ukraine's borders as it builds toward a possible invasion early next year.

After speaking with Zelenskyy, Biden briefed leaders of nine NATO members in Eastern Europe, including three former Soviet republics. It's part of weeks of coordination with NATO allies on the response to the Russian military buildup on Ukraine's border.

Before and after his call with Putin, Biden spoke with the leaders of Britain, France, Germany and Italy. France and Germany took the lead in brokering the 2015 peace deal between Ukraine and the Russia-backed rebels, in what's known as the Normandy format.

"We hope by Friday we're gonna be able to say, announce to you, we're having meetings at a higher level," Biden said Wednesday. "Not just with us, but with at least four of our major NATO allies, and Russia."

The meetings would address "the future of Russia's concern relative to NATO writ large, and whether or not we could work out any accommodations as it relates to bringing down the temperature" in Ukraine's east," Biden said.

Speaking at a news conference in Paris on Thursday, French President Emmanuel Macron defended French-German diplomatic efforts over the past several years to mediate between Russia and Ukraine, but said he welcomed Biden's outreach to his European allies on the matter.

"Our aim – all of us Europeans and our American partners -- is first to be at Ukraine's side to ensure its security ... and to start a demanding dialogue with Russia," Macron said.

Asked about any need for Ukrainian compromises, Sullivan told reporters Tuesday that "Ukrainians have come forward with constructive ideas for how to move the diplomacy forward. We're encouraging that."

Under the 2015 deal, Ukraine agreed to change its constitution to accommodate the "peculiarities" of the two Donbas separatist republics and to legalize their "special status." Some analysts said the deal's vagueness, and some conflicting requirements, make its provisions effectively unworkable.

Ukraine is willing to engage in talks on defining "special status," including possible changes that account for the cultural and linguistic differences of its eastern Donbas region, which has a higher proportion of native Russian speakers, the person familiar with the private talks between Ukraine and the United States said.

But Ukraine would reject any change that gives the region virtual veto power over national policy, the person said.

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Associated Press writers Robert Burns and Aamer Madhani in Washington contributed to this story.