Russian-US tensions over Ukraine exploded into a public row Monday, as secretary of state John Kerry refused to attend talks in Moscow after his counterpart snubbed Kiev's interim leaders.
Russia's wily foreign minister, Sergei Lavrov, came out swinging first, putting the much-travelled Kerry on the spot by denouncing him for turning down an invitation to meet on Moscow on Monday.
But US officials hit back that the time was not right for Kerry to visit Moscow as there was no sign that President Vladimir Putin was prepared to negotiate, or that Lavrov was authorized to lead any talks.
Washington has led global efforts to defuse tensions since Russian troops deployed troops last week in Ukraine's Crimea peninsula after a months-long tussle over Ukraine's future direction, which saw the pro-Moscow president Viktor Yanukovych flee the country.
Read: Shots fired in air during raid at Crimea naval base
The Crimea, home to Moscow's Black Sea Fleet, will hold a potentially explosive vote on Sunday on whether to split from Ukraine and join Russia -- threatening to tear the country apart.
Kerry handed over a page-long set of proposals on a diplomatic way forward to Lavrov when the two men met in Paris and Rome last week.
The proposals, which include setting up a contact group to bring together Ukraine and the Russia in formal talks, were refined after the two men talked by phone on Saturday.
Washington was still waiting for Lavrov to come back to Kerry on those ideas, US officials insisted.
But in a televised meeting Monday with Russian President Vladimir Putin, Lavrov said Kerry's ideas "do not suit us very much" and that his US counterpart had changed his mind after initially agreeing to come to Moscow.
The Russian government, he said, will now make a series of counter-proposals "on the basis of international law and take into account the interests of all Ukrainians without exception."
State Department spokeswoman Jen Psaki insisted Kerry is prepared to take part in talks "if and when we see concrete evidence that Russia is prepared to engage."
The US wants to see an end to Russian military activities in southern Crimea and for Moscow to enable independent observers from the OSCE to reach the area.
"We want to see a halt in the drive for annexation of Crimea," Psaki said.
Psaki bluntly rejected any suggestion that Kerry was deliberately spurning Lavrov.
"He never shies away from hopping on a plane or having an in-person meeting, but we want to ensure that that is undertaken with seriousness on the other end as well," Psaki said.
Interim Ukraine PM to visit US
The global community is seeking to shore up the government of interim Ukrainian prime minister Arseniy Yatsenyuk, who is due to visit the White House on Wednesday.
US officials did not rule out that Kerry could travel to Moscow later this week should Putin and Lavrov appear ready to talk seriously.
But Moscow is refusing point blank to talk to the interim leaders, maintaining they are illegitimate.
"The point is we're not just going to walk into something, where they're just going to say 'no, no, no' to everything and we've travelled all the way there. We're not going to do that," a State Department official said, asking not to be named.
President Barack Obama's welcome for Yatsenyuk shows the United States "believes the Ukrainian government has responsibly filled the vacuum left by the sudden, hasty and voluntary departure" of Yanukovych, White House spokesman Jay Carney said.
In his latest calls to prominent world leaders on the crisis, Obama spoke Monday to both President Nursultan Nazarbayev of Kazakhstan and Spain's Prime Minister Mariano Rajoy.
In the call with the Kazakh leader, the two men talked of finding "a peaceful resolution to the dispute between Russia and Ukraine," the White House said.
"They agreed on the importance of upholding principles of sovereignty and territorial integrity," it added in a statement.
Kazakhstan is a key potential member of the Eurasian Economic Union that Putin hopes to set up with ex-Soviet states. But Yanukovych's ouster and Ukraine's expected turn towards the West leaves the plan for the bloc -- supposed to compete with the EU -- in doubt.