Gunfire could be heard from the airfield at the town of Kramatorsk in eastern Ukraine; Ukrainian troops were seen disembarking from helicopters.
European Union diplomats said there was little prospect of a special summit to discuss additional sanctions because positions are so far apart.
The most serious East-West crisis since the end of the Cold War shows no signs of de-escalating.
Russia's intervention in Ukraine has Georgians worried about another Russia move - this time to the critical Caucuses land bridge.
Separatist groups that favor Russia have taken over Ukrainian government buildings in Donetsk, Lugansk, Kharkov and Slavyansk.
The interim government has yet to follow through with promises to use military might to retake eastern Ukraine.
Rebels in the eastern Ukraine town of Slaviansk Monday issued a bold call for Russian President Putin to help them.
Earlier, Russia's Deputy Premier Arkady Dvorkovich went to China as part of a delegation to discuss cooperation in the energy sector.
The State Department refuted 10 claims made by Russia about the ongoing Ukraine crisis in a harshly worded statement.
In what is being called an "act of aggression," pro-Russian activists seized government buildings and a police station, witnesses said.
President Putin also said Russia has no plans to halt deliveries to Ukraine, but it might make Kiev pay up front.
The U.S. said it's taking steps to assist Ukraine, including the provision of emergency finance and technical assistance in energy security.
Russia energy giant Gazprom says Ukraine owes $2.2 billion for gas supplies and missed the deadline for paying for its March supplies.
The capital flight coincides with slumping investment and a sharp deterioration in business confidence.
Officials on both sides of the Ukraine crisis are pointing to each other for escalating tensions in Eastern Ukraine.
Buildings were also seized in Kharkiv and Donetsk, where pro-Russia demonstrators called for a “people’s city council.”
Ukrainian officials have accused Russia of sending FSB operatives, and explosives, into the country to stamp out anti-government protests.
Move aims to ease tension after Moscow accused the bank of illegally blocking the transaction.
NATO cites key signs at the border indicating that Russia may be ready to pounce.
Russia's continued troop movements on Ukraine border worry NATO commander
A Russian news organization reported that Ukraine had asked to resume normal relations with Russia once again.
In a statement, the Russian Foreign Ministry suggested the action would have consequences for the U.S. embassy and consulate in Russia.