Ukraine crisis: US condemns Russian 'aggression'
The US has accused Russia of an "act of aggression" over its alleged deployment on Ukraine's Crimean peninsula.
President Barack Obama said Russian President Vladimir Putin's assertion that no troops had been mobilised was "not fooling anyone", and that Russia had no right to intervene in Crimea.
Secretary of State John Kerry, visiting Kiev, praised the "restraint" of the new government.
Moscow is now in de facto control of Crimea, where tensions are high.
Troops in what appear to be Russian uniforms are surrounding Ukrainian military bases and other installations, while two Ukrainian warships are reported to be blocked by a Russian ship in the port of Sevastopol.
In one hint of progress on Tuesday, Ukraine's new Prime Minister Arseniy Yatsenyuk said tentative talks had taken place between Russian and Ukrainian ministers. He described them as "rather timid" but "first steps", AFP news agency reports.
Mr Yatsenyuk has insisted that there are no grounds for Russian troops to be in Ukraine. Kiev was still prepared to build a new post-uprising relationship with Moscow, he was quoted as saying by the Ukrainian news agency Unian.
'Pretext for invasion'
Earlier, Mr Putin said Ukraine was in "chaos" having been seized by "nationalists" and "anti-Semites", and that Russia had a right to protect Russian citizens in the country.
But Mr Obama said the emergence of Russian troops from their barracks in Crimea was not about concern for Russian nationals but "Russia seeking through force to exert influence on a neighbouring country".
"That is not how international law is supposed to operate," he said.
In Kiev, Mr Kerry also dismissed Mr Putin's denial of troop deployments.
"I think that it is clear that Russia has been working hard to create a pretext for being able to invade further," he said.
"Russia has talked about Russian-speaking minority citizens who are under siege... They're not. It is not appropriate to invade a country and dictate what you want to achieve at the end of the barrel of a gun."
Mr Kerry was speaking after holding talks with interim President Olexander Turchynov, Prime Minister Yatsenyuk and other new government figures.
He praised them for acting "remarkably responsibly by urging total calm, by not wanting to have any provocation".
In other diplomatic developments on Tuesday
- Nato and Russia agreed to hold talks on Wednesday. Nato General Secretary Anders Fogh Rasmussen said Russia continued to "violate Ukraine's sovereignty and territorial integrity", presenting "serious implications for the security and stability of the Euro-Atlantic area"
- EU foreign policy chief Catherine Ashton had "a useful discussion" with Russian Foreign Minister Sergei Lavrov in Madrid, said an EU source
- Mr Lavrov said the Russian position was "honest" and would not change, and that sanctions against Russia would be "counter-productive"
Both the US and the EU have offered financial help to Ukraine, which is facing a growing economic crisis amid its severed ties with Moscow.
Mr Kerry took to Kiev a $1bn (£600m; 720m euros) package of energy subsidies, and told the crowds who remain in Independence Square after months of protests that President Obama "is planning more assistance".
The EU is considering paying the $2bn which Ukraine owes to Russia in gas bills, AFP news agency quotes EU Energy Commissioner Guenther Oettinger as saying.