WASHINGTON, Jan 27 (Reuters) - The United States on Thursday called on China to use its influence with Russia to urge a diplomatic solution to the Ukraine crisis, but policy experts doubted Beijing would back Washington in the standoff.

China's Foreign Minister Wang Yi spoke by phone with U.S. Secretary of State Antony Blinken and Beijing said it wanted all sides to remain calm and "refrain from doing things that agitate tensions and hype up the crisis."

Blinken stressed that tensions should be reduced and warned of the security and economic risks from any Russian aggression, the State Department said.

U.S. Under Secretary of State for Political Affairs Victoria Nuland said U.S. messages to Beijing had been very clear.

"We are calling on Beijing to use its influence with Moscow to urge diplomacy, because if there is a conflict in the Ukraine it is not going to be good for China either," Nuland said at a regular State Department news conference. "There will be a significant impact on the global economy. There will be a significant impact in the energy sphere."

China's U.N. ambassador Zhang Jun said a "time-honored Olympic Truce" for the Beijing Winter Games that begin on Feb. 4 would start from Jan. 28.

"Let's take this opportunity to promote peace, solidarity, cooperation, and other common values shared by all humanity, to make our world a better place," Zhang tweeted.

Russia has been building up its forces on Ukraine's borders for months and has demanded NATO pull troops and weapons from eastern Europe and bar Ukraine, a former Soviet state, from ever joining the U.S.-led military alliance.

NATO allies reject this but say they are ready to discuss arms control and confidence-building measures.

Daniel Russel, the senior U.S. diplomat for Asia under former President Barack Obama, said that while China could not be happy about the possibility of an invasion of Ukraine on the eve of the Olympics, "Wang Yi chose to defend Russia's 'legitimate security concerns' rather than offer any support to Blinken."

Bonnie Glaser of the German Marshall Fund of the United States said Beijing could act as a spoiler to any attempts by the United States and its allies to impose costs on Russia.

"It is unlikely that the U.S. can get China on board over Ukraine. Beijing won't endorse use of force, but it is sympathetic with Russian views of NATO. And this is not just about the Olympics," Glaser said.

If the United States and the European Union imposed sanctions on Russia, "China is likely to take steps to mitigate their impact," she said.