VIENNA (AFP) - Washington hopes any fallout from the Ukraine crisis will not undermine unity between the US, Russia and other powers in ongoing nuclear talks with Iran, a senior US administration official said Friday.

"I think that we all hope that the incredibly difficult situation in Ukraine will not create issues for this negotiation," the official said ahead of the next round of talks between Iran and six world powers in Vienna starting Tuesday.

"We hope that whatever happens in the days ahead, whatever actions we and the international community take, depending upon the decisions and the choices that Russia makes, that any actions that Russia subsequently takes will not put these negotiations at risk," the official said.

The official added that so far in the current push to resolve the Iranian nuclear crisis, which saw a major but interim deal struck in November, the United States, China, Russia, Britain, France and Germany have been "completely united".

"This is a very cohesive, very well coordinated effort and even when there are differences, which there are, we bridge those differences and everyone is very focused on what is necessary," the official said.

Under November's interim deal, Iran froze certain nuclear activities for six months in exchange for relief from UN and Western sanctions hitting its economy hard.

Meeting in Vienna from Tuesday for the second time since November, the six powers and Iran aim to transform the interim accord into a long-lasting "comprehensive" agreement by the time it expires on July 20.

Russian Foreign Minister Sergei Lavrov said Friday after three hours of talks with US counterpart John Kerry in London that Moscow and the West still had "no common vision" on Ukraine.

Kerry has warned Russia that Washington and Europe could announce a "very serious" response as early as Monday if Moscow does not pull back the troops who seized control of Crimea days after the pro-Kremlin regime fell in Kiev last month.