GARDEZ, Afghanistan ? U.S. troops scoured caves and cleared ridges of al-Qaida diehards Thursday, but sandstorms and high winds grounded helicopters and threatened to disrupt the U.S.-led air and ground offensive.

After some of the heaviest bombing in the six-day offensive, a number of supply flights were delayed or canceled because of the worsening weather. U.S. officials acknowledged pilots and troops on the ground would have a harder time routing the fighters in such bad conditions.

Maj. Bryan Hilfery, spokesman for the 10th Mountain Division, said 100 militants were killed Wednesday. Allied attacks also destroyed some of their heavy weaponry which includes mortars, small cannons, rocket-propelled grenades and shoulder-fired anti-aircraft missiles.

“We’re continuing to bolster our efforts, and units are continuing to maneuver in fire today, clearing ridgelines, caves and pockets of al-Qaida resistance,” Hilfery said at Bagram air base, north of the Afghan capital Kabul.

U.S. officials and Afghan commanders said al-Qaida sympathizers including some from Pakistan had crossed into the mountains to join the fight. Afghans said enemy forces may now number 1,000.

The commanders insisted the routes to the mountain passes had since been sealed even though Taliban fighters managed to bring some of their slain comrades to the foothills of Surmad for burial Tuesday. Surmad is 18 miles south of Gardez, the capital of Paktia Province. Gardez is about 75 miles south of Kabul, the capital.

U.S. officials have said hundreds of fugitive fighters have been killed since Operation Anaconda began and small numbers detained. Eight American and three Afghan troops have died in the offensive.

Five international peacekeepers were killed Wednesday when a Soviet-era missile they were trying to defuse exploded, the first fatalities in the force. And on Thursday in Kandahar, a fire at an ammunition depot near the coalition base killed three U.S.-allied Afghan fighters. Canadian officials said the Afghans may have tripped a booby trap, sparking a fire.

New troops were headed to the region, including about 200 soldiers from the 101st Airborne Division, equipped with 16 Apache helicopters and four CH-47 Chinook; and 107 members of a Canadian infantry unit rotating in.