WASHINGTON (AFP) - Two Russian fighters entered a United States "air defence identification zone" two days ago and were intercepted by American F-22 jets near Alaska, military officers said on Friday.

The incursion on Wednesday was followed by a second incident on Thursday involving two Russian long-range bombers, which flew into Canada's air defence identification zone (ADIZ) and were intercepted by two Canadian F-18 jets, officers said. In both cases the Russian aircraft flew out of the area without incident.

The Russian warplanes "never entered US sovereign air space" or Canadian air space, said Major Jamie Humphries, a spokesman for the North American Aerospace Defence Command. In Wednesday's encounter near Alaska, the Russian fighters were accompanied by two refueling tankers and two long-range bombers, he said.

Although Russian aircraft have entered the zone previously it was "the first time in a long time" that fighter jets passed through the area, said a US defence official, who spoke on condition of anonymity.

To safeguard a country's air space, air defence identification zones extend beyond territorial air space and are designed as a buffer to give a government more time to respond to potentially hostile aircraft. But the zones do not fall under international treaties and are not regulated under international law.

The Russian aircraft flights coincided with a visit by Ukraine's President Petro Poroshenko to Washington, where he made an impassioned address on Thursday before a joint session of the US Congress, denouncing Russia's military intervention in his country.

But Pentagon spokesman Rear-Admiral John Kirby said there was no indication of a link between Poroshenko's visit to Washington and the air incidents.

"We've faced these kinds of incidents before. We take them very, very seriously. And we routinely intercept them," Rear-Adm Kirby told CNN.

"We'll make our intentions known to Russia as we always do and we'll certainly discuss our concerns with them at the appropriate time and in the appropriate venue." It was unclear if the Russian aircraft were in the area due to exercises announced by Moscow in far-eastern regions, including the off-shore naval training grounds of the Kamchatka region.

The Vostok-2014 exercise started on Friday and was scheduled to last through September 25 and included 100,000 troops and 120 aircraft, according to the Russian defense ministry.

In Wednesday's encounter near Alaska, the Russian aircraft veered into the air defense zone at about 7pm local time. In Thursday's episode near Canada, the Russian bombers flew into the area at about 1.30am local time.