NATO has said it will start reconnaissance flights over Poland and Romania to monitor the situation in neighbouring Ukraine where Russian forces have taken control of Crimea.
The AWACS (Airborne Warning and Control System) flights will take place solely over NATO territory, the alliance said on Monday.
Ukraine is not a NATO member but Russia's intervention in Crimea has alarmed neighbouring countries, including alliance members that used to be dominated by the Soviet Union.
The announcement comes as the United States is sending a dozen F-16 fighter jets and 300 service personnel to Poland as part of a training exercise.
The US and Russia are bitterly divided over how to ease the crisis, with each challenging the other to show they are really interested in a peaceful outcome.
Russia denounced alleged lawlessness by far-right activists in eastern Ukraine on Monday, in a statement likely to trigger fear in Ukraine over possible Russian intervention.
|Tensions rise in pro-Russia areas of Ukraine
The Russian Foreign Ministry said it was outraged by lawlessness in the country’s east, blaming the group Right Sector for "conniving" with the new government in Kiev.
Right Sector is a grouping of several far-right and nationalist factions who were actively involved in the uprising against Moscow-backed President Viktor Yanukovich.
The movement uses swastika signs, is blamed for various attacks, and according to its leader Dmitro Yarosh, has “enough weapons to defend all of Ukraine”.
Its activists were among the most radical and confrontational of the demonstrators in the Ukrainian capital, Kiev, and organised “self-defence” brigades for the protest camp.
Witnesses in eastern Ukraine told Reuters news agency that tensions had been fuelled by pro-Russian activists to provide Russian President Vladimir Putin with a justification for invading Ukraine to protect Russians.
Pro-Russia sentiment is at a high and there are fears Russia could seek to incorporate that area as well.
Violence against Russians
A referendum has been called in Crimea for March 16 on whether the region should split off and seek to become part of Russia.
Russia has criticised the West for being silent over violence and detentions taking place against Russians, highlighting the attack on a pro-Moscow demonstrators in the eastern city of Kharkiv on March 8 and the detention of Russian journalists.
"The shamefaced silence of our Western partners, human rights organisations and foreign media is surprising. It raises the question, where is the notorious objectivity and commitment to democracy?" it said.
Ukraine's government and Western leaders have accused Russian officials and media of distorting the facts to portray the protesters who Yanukovich's rule as violent extremists.
On Wednesday, Ukrainian Prime Minister Arseniy Yatsenyuk will be received in Washington by President Barack Obama.
Obama has warned that the March 16 vote in Crimea would violate international law. But on Sunday, Putin made it clear that he supports the referendum, in phone calls with German Chancellor Angela Merkel and British Minister David Cameron.
“The steps taken by the legitimate leadership of Crimea are based on the norms of international law and aim to ensure the legal interests of the population of the peninsula,” said Putin, according to the Kremlin.
Putin has refused to have any dealings with the new Ukrainian leaders who replaced fugitive pro-Kremlin Yanukovich.