Britain said it had scrambled Typhoon fighter jets to intercept seven Russian military aircraft flying near the Baltic states, as tensions between the West and Russia over Ukraine remained high.
A NATO official said the Russian aircraft were armed and in the vicinity of NATO airspace.
They did not file a flight plan and did not use their transponders, making them invisible to civilian air traffic control and a safety hazard to commercial air traffic, which was why NATO had intercepted them, he said.
The alliance last month tripled the number of fighter jets based in the Baltics as part of measures to beef up its defenses in eastern Europe following Russia's annexation of Crimea. Britain contributed four Typhoons to the operation.
Britain's Ministry of Defense said Wednesday it had launched the Typhoons on Tuesday after four separate groups of aircraft were detected in international airspace near the Baltics by NATO air defenses.
It said the aircraft had been identified as a Russian Tupolev Tu-22 bomber, four Sukhoi Su-27 fighters, one Beriev A-50 early warning aircraft, and an Antonov An-26 transport aircraft.
They appeared to be carrying out routine training, it said.
"The Russian aircraft were monitored by the RAF Typhoons and escorted on their way," the ministry said in a statement.
NATO planes patrolling the Baltics have scrambled repeatedly in the last week in response to sightings of Russian military aircraft.
Last Thursday, Latvia said NATO fighters had scrambled a record four times after a total of 16 Russian military planes were spotted close to the country's air space.
Local news agency BNS quoted Latvia's Prime Minister Laimdota Straujuma on Monday as saying that the increased activity by Russian military planes was an "intimidation tactic" and a demonstration of power by Russia. "The same tactics are also used in Ukraine and elsewhere," she said.
The incidents may have been connected to military exercises in Russia's Baltic exclave of Kaliningrad that Moscow said last week it was holding in response to drills by NATO allies in parts of eastern Europe.