Fighters scramble after Russian jets

• U.S. official: Incident related to visit by Ukraine president


• NORAD says six Russian aircraft flew about 55 nautical miles from Alaska coastline


• Two Russian planes flew about 40 miles off Canadian coastline, NORAD says

(CNN) -- Two U.S. jets this week intercepted six Russian planes that neared U.S. airspace off Alaska and Canadian planes intercepted two Russian bombers that approached Canadian airspace, NORAD reported.

A U.S. official told CNN's Barbara Starr that officials in Washington think the incidents were related to the visit by Ukrainian President Petro Poroshenko, who talked to Canadian officials Wednesday and President Barack Obama on Thursday. The United States promised Ukraine $46 million in nonlethal aid for its battle with pro-Russian militants.

An American RC-135 Rivet, like the one seen here, was forced to enter Swedish air space after a Russian radar system locked onto it.

A U.S. Air Force RC-135U aircraft, like the one pictured here, was buzzed by a Russian fighter jet near Japan earlier this year. A top U.S. Air Force general said Russia was stepping up its military activities in the Asia-Pacific region as tensions increased over the Ukraine and Russia's move into Crimea.

A Russian Air Force flight demonstration team perform with their Su-27 jet fighters over St. Petersburg, Russia. An Su-27 "showed its belly" to a U.S. reconnaissance jet in April.

A U.S. Air Force F-22 Raptor, top, escorts a Russian Air Force Tu-95 bomber off the coast of Alaska during 2011.

Russian Air Force Tu-95 bombers fly over Moscow's Red Square on May 7.

An F-15 Eagle from the 19th Fighter Squadron takes off at Joint Base Elmendorf-Richardson in Alaska. F-15 fighters intercepted Russian bombers off California in June.

A B-52, left, and a B-2 stealth bomber fly in formation over Shreveport, Louisiana. The United States deployed both types of aircraft to Britain in June.

The Spirit of Ohio, a B-2 bomber, takes off from Whiteman Air Force Base in Missouri. Two B-2s were deployed to England in June.

A B-52 lands at an airbase in England on June 4. The B-52 was one of three to arrive.

U.S. and Russian military aircraft

U.S. and Russian military aircraft

U.S. and Russian military aircraft

U.S. and Russian military aircraft

U.S. and Russian military aircraft

U.S. and Russian military aircraft

U.S. and Russian military aircraft

U.S. and Russian military aircraft

U.S. and Russian military aircraft

Photos: U.S. and Russian military aircraft

On Wednesday evening, two Alaskan-based F-22 fighter jets intercepted two Russian IL-78 refueling tankers, two Russian Mig-31 fighter jets and two Russian Bear long-range bombers, according to Capt. Jeff Davis of the North American Aerospace Defense Command (NORAD).

More than six hours later, early Thursday morning, two Canadian CF-18 fighter jets intercepted two Russian Bear long-range bombers in the Beaufort Sea, Davis said. Those Russian planes came within about 40 nautical miles of the Canadian coastline, he said.

Though the planes did not enter sovereign territory, the statement said, they did enter the U.S. Air Defense Identification Zone west of Alaska and the Canadian ADIZ, according to a statement.

U.S. official: Spy plane flees Russian jet, radar; ends up over Sweden

The ADIZ is a zone of airspace which extends approximately 200 miles from the coastline and is mainly within international airspace, according to the statement. The outer limits of the ADIZ go beyond U.S. sovereign air space.

State aircraft of sovereign nations are not required to file flight plans. NORAD said it has intercepted more than 50 Russian aircraft in the last five years. NORAD jets usually intercept one or two Russian planes, normally bombers.

Dutch fighter jets intercept 2 Russian bombers in their airspace