NATO is to deploy reconnaissance planes in Poland and Romania to monitor the Ukrainian crisis.
It gave the go-ahead for the flights on Monday, a NATO spokesman said.
“All AWACS [Airborne Warning and Control System] reconnaissance flights will take place solely over alliance territory,” the official said.
It comes as Russia cements its control of Ukraine’s Crimea ahead of Sunday’s referendum to join Russia. Ukraine and the West say this is illegal.
The Awacs have some capacity to monitor vehicles on the ground but their chief mission will be reassurance. The flights will be mounted from bases in Germany and the United Kingdom.
An Alliance spokesman described the decision as “an appropriate and responsible action in line with Nato’s decision to intensify our ongoing assessment of the implications of this crisis for Alliance security”.
The US has already sought to reassure its allies in Europe by adding six additional F-15 aircraft to the NATO air policing mission over the Baltic Republics and it is also despatching 12 F-16 fighters for a training exercise in Poland.
In the latest move on Monday, armed men – said to be Russian troops and local militias – seized a military hospital in Crimea.
The attackers marched into the hospital in the regional capital Simferopol, threatening staff and some 30 patients.
Pro-Russian troops are also blockading Ukrainian troops across Crimea, which is an autonomous region.
Moscow has officially denied that its troops are taking part in the blockades, describing the armed men with no insignia as Crimea’s “self-defence” forces.
The government in Kiev – as well as the US and EU – accuse Russia of invading Ukraine, in violation of international law.
NATO said the surveillance flights would “enhance the alliance’s situational awareness”.