Canadian CF-18 fighter jets helped plug a hole in U.S. air defences for almost two weeks this month after American jets were grounded as part of a crash investigation.

The request to fill in for U.S. F-15s over the Alaskan coast was considered an urgent priority for NORAD, especially in light of the return of Russian strategic bombers to Arctic patrols.

Although not unprecedented, defence officials said the now-concluded operation was one of those "extremely rare" occasions when Canada was able to contribute to the defence of its much larger neighbour.

The aircraft are now back at their home base in Bagotville, Que., and the air force was able to lift what was described as a veil of operational security.

"I can't say precisely how many CF-18s were involved, but certainly there were a few," said Maj. Mike Lagace, a spokesman for Canadian NORAD operations, based in Winnipeg.

"We really don't want to say very much in case they're called on again."

On Nov. 2, an American F-15C — an older variant of the hardy fighter-bomber — suffered a catastrophic failure over Missouri and crashed, resulting in the grounding of 700 front-line aircraft. The pilot ejected to safety.

Urgent inspections were carried out on the entire F-15 fleet, with newer models eventually restricted to "mission-critical" sorties only.

U.S. Air Force Gen. John D. W. Corley, who heads Air Combat Command, said in a statement that the grounding had "significant operational impact" but that U.S. and coalition partners were able to make up the difference.

In Afghanistan, French Mirage 2000 jet fighters replaced American F-15s in providing close air support for NATO, including Canadians troops.

Japan also grounded its F-15s following the U.S. crash.

F-15s have been a pillar of U.S. air power since the mid-1970s, but the air force said in 2004 that it intended to replace them gradually with Lockheed Martin's modern F-22 Raptor.

Canadian jets were stationed in Alaska

NORAD, the joint American-Canadian air defence command, had initially hoped to be able to fill the Alaskan gap with F-22s, but not enough of them were available, said Canadian defence officials.

The Canadian fighter jets were stationed at Elmendorf Air Force Base, near Anchorage, Alaska, and worked alongside the American 611th Air Operations Squadron, conducting sovereignty patrols on behalf of the Americans.

"It shows the joint capability where Canadian and American forces work as one," said Lagace.

The resumption of Russian Tu-95MS bomber flights this summer along the Alaskan and Canadian Arctic borders have kept NORAD "quite busy" and the pressure has not eased, he said.

After almost a decade of infrequent forays, the Russians startled Western militaries by resuming High Arctic long-range patrols, which had been a prominent feature of the Cold War.

Flying in pairs for up to 12 hours, the Tu-95MS strategic bombers trace the edge of American, Canadian and often Danish air space near Greenland, forcing NORAD fighters to scramble to meet them.