Armed men patrol at the airport in Simferopol, Crimea February 28, 2014. [Photo/Agencies]

In Washington, the White House urged Russia to respect Ukraine's territorial integrity and sovereignty.

"We strongly support Ukraine's territorial integrity and sovereignty," spokesman Jay Carney told reporters at a regular press briefing. "We expect other nations to do the same."

Carney said Washington was "closely watching" Russia's military exercises along the Ukrainian border.

"We expect Russia to be transparent about these activities and to avoid provocative actions," he said. "We urge them not to take any steps that could be misinterpreted or lead to a miscalculation during a very delicate time."

US Secretary of State John Kerry, who spoke to his Russian counterpart Sergey Lavrov over the phone, also said he was assured of Russia's respect for the sovereignty of Ukraine.

"We believe that everybody now needs to step back and avoid any kind of provocations," the top American envoy said.

In Brussels, NATO Secretary-General Anders Fogh Rasmussen urged Russia not to take any action that could escalate tension or create misunderstanding in Ukraine.

"NATO has a long-standing partnership with Ukraine. Today we made clear that NATO stands ready to support democratic development, defence reforms, military cooperation and democratic control over the security sector," said Rasmussen.

Also on Thursday, Russian news agencies reported that Russian President Vladimir Putin has ordered the government to consider humanitarian assistance to the Russian-speaking region of Crimea and talk to the West about bailing out Ukraine.

Following Western calls on the Kremlin to join efforts to help Ukraine avoid a default, Putin asked the government to talk to the International Monetary Fund (IMF) and others about financial aid for Ukraine.

The move, the first reaction of the Russian president to the Ukraine crisis, was announced by his spokesman Dmitry Peskov in a statement. No further details were provided except that assistance to Crimea could be offered by some Russian provinces.

Yanukovych still considers himself Ukraine head	Armed men seize govt HQ in Ukraine's Crimea	New Ukraine ministers proposed, Russian troops on alert