President Barack Obama held "constructive" talks Friday with Russian President Vladimir Putin as he pressed for swift implementation of a deal to end Ukraine's deadly crackdown on protesters.

The phone call came at a prickly time in US-Russia relations with both sides at odds over Ukraine, Syria and other issues, but also after Obama took pains to reject the idea that a new Cold War-style confrontation was brewing.

A senior US official described the call as "constructive," but also warned that the agreement reached in Kiev between the government and top opposition leaders was "very, very fragile."

Putin and Obama agreed that the Ukraine agreement needed to be swiftly implemented and that all sides needed to refrain from violence, as they also pressed for the need to stabilize the economy.

The call touched on other issues as well, including Syria, where Russia backs President Bashar al-Assad, and the Sochi Winter Olympics.

Washington had offered staunch support for protesters in Kiev, demanding political concessions from the Moscow-backed government of President Viktor Yanukovych and had warned of "consequences" if violence, which has killed 100 people, did not stop.

The White House gave an initial welcome to the deal and praised European Union diplomats who helped to broker it.

- 'Courageous' opposition leaders -

White House spokesman Jay Carney said the deal was "consistent with what we have advocated in calling for a de-escalation of the violence, constitutional change, a coalition government and early elections."

"We call for immediate implementation of the initial steps -- an end to the violence, amnesty and security normalization, and passage of the constitutional package in (parliament) -- to provide space for the negotiations to begin on formation of a technocratic coalition government," Carney said.

Vice President Joe Biden had called Yanukovych on Thursday, warning that the United States was ready to impose sanctions if he did not halt attacks by his security forces on demonstrators which have killed nearly 100 people.

Yanukovych's dramatic decision to hold early elections and form a new unity government was met with caution by tens of thousands gathered on central Kiev's main square.

The deal was signed in the presence of EU envoys, Yanukovych and three top opposition leaders who included boxer turned lawmaker Vitali Klitschko.

A representative for Putin missed the meeting. The White House statement, however, said the deal had been "witnessed" by Russia.

Obama on Wednesday insisted that the United States did not see its many disputes with Russia as symptomatic as a struggle over a Cold War-style "chessboard."

Carney expanded on the president's remarks, stressing that "this is not about the United States and Russia or the West and Russia -- this is about Ukraine and the Ukrainian people and their desire for the right to choose their own destiny."

Separately, a US official said that his deputy William Burns plans to visit Kiev early next week, and that Assistant Secretary of State for European and Eurasian Affairs Victoria Nuland would likely travel there in early March.

"We anticipate being part of the group of countries that support the implementation going forward," the official added.

The official said Yanukovych was visiting Kharkiv, considered his eastern political base, for a meeting, while playing down suggestions the president had fled the capital.

And Ukrainian Defense Minister Pavlo Lebedev told Pentagon chief Chuck Hagel earlier that his country's army would not use force against protesters in Kiev, the US Defense Department said.