US President Barack Obama and Russian President Vladimir Putin agreed on Friday that a deal aimed at halting bloody clashes between government forces and protesters in Ukraine needs to be implemented quickly so that the country stabilizes, a US official said.The two leaders spoke by phone after Ukrainian President Viktor Yanukovich and opposition leaders signed a European Union-mediated peace deal."They agreed that the agreement reached today needed to be implemented quickly, that it was very important to encourage all sides to refrain from violence, that there was a real opportunity here for a peaceful outcome," a senior US State Department official told reporters on a conference call.The White House said details of the agreement are consistent with what the United States had been urging, such as a de-escalation of the violence, constitutional change, a coalition government and early elections.The State Department official warned, however, that the deal remains "very, very fragile," and said international support will be needed to help stabilize the country."This has been a very tough sell and will continue to be a tough sell for the opposition to make to those on the streets. This is not least because of the horrible, horrible violence of the last two days," the official said.Tony Blinken, deputy national security adviser, said in a CNN interview that the Obama administration had made clear to Ukraine there would be consequences if the violence continued."And I think that had an important impact in getting people to move," Blinken said. "We've already issued some visa restrictions on those who were responsible for the violence and repression."We also told them that other steps could be forthcoming and I think that had a real impact on their thinking. Not just folks in the government, but some of the strong oligarchs who support the government," he added.US Deputy Secretary of State William Burns will go to Kiev early next week and Assistant Secretary of State for Europe Victoria Nuland will likely visit in early March to be part of international support for the implementation process.Senior US officials had been preparing new sanctions to impose on Ukraine's government after dozens of people were killed in Kiev during mass demonstrations this week.The White House reiterated that those responsible for the violence must be held accountable."We are not ruling out sanctions to hold those responsible for the violence accountable, especially should there be further violence or violation of the agreement," said White House spokesman Jay Carney.Carney said efforts of the French, Polish and German foreign ministers as well as US leaders helped bring about the deal. He added that "Russia witnessed the agreement and ... played an important role in that respect.""It is in Russia's interest that Ukraine not be engulfed in violence - Kiev or other places - and that it return to stability, and that progress be made toward a future in Ukraine that reflects the will of the Ukrainian people," Carney told a news briefing."So it's very important to view this not as a tug-of-war between East and West or the United States and Russia," he added.