WASHINGTON - More than 100 U.S. special operations forces, including Army Rangers, conducted a nighttime raid on Afghanistan, the first known U.S. ground action of the military campaign, a U.S. official said last night.

There were no confirmed reports of American casualties on the ground. But the Pentagon said last night that a U.S. military helicopter crashed in an accident yesterday in neighboring Pakistan, killing two service members.

A U.S. official said the helicopter was not directly involved in the ground operation in Afghanistan but was on standby for search-and-rescue missions if needed by the special operations troops. The two service members killed, who were not identified, are the first known combat-related U.S. casualties in the region.

President Bush, after meeting with Japanese Prime Minister Junichiro Koizumi at an economic summit in Shanghai, China, said he grieved for the soldiers and their families.

"The important thing for me to tell the American people is these soldiers will not have died in vain," he said.

Bush refused to comment on the raid but said of the U.S. operation in general: "I am satisfied we are making very good progress. We are dismantling the Taliban defenses. ... We are destroying terrorists' hideaways. We are slowly but surely circling the terrorists so we can bring them to justice."

The U.S. special forces completed the operation in Afghanistan within a few hours early today local time, said the U.S. official, who spoke on condition of anonymity. The helicopter-borne forces then left Afghan airspace, the official said, presumably to return to base.

It could not be immediately determined what the commandos were attempting to do in Afghanistan.

Some news reports indicated that the commandos engaged in ground combat around the Taliban stronghold of Kandahar in southern Afghanistan, opening a new phase of the war on terrorism after nearly two weeks of punishing airstrikes. The Kandahar area has been a primary target of U.S. aerial assaults in recent days.

It was not clear where the U.S. special forces had originated. NBC News reported that the commandos came in on helicopters from the USS Kitty Hawk, a carrier in the Arabian Sea that is known to be carrying special operations troops. Other reports said that some of the helicopters came from land bases in the region and that fighter aircraft had provided cover for them.

In recent days, Taliban leaders had dared the United States to launch a heavy ground campaign. Sohail Shaheen, a spokesman for the Taliban embassy in Pakistan, said:

"If they want to send in soldiers, they should send in 100,000. Then it can be a fight between our soldiers and theirs."

Last night, Sen. Carl Levin, chairman of the Senate Armed Services Committee, said that introducing U.S. commando forces into Afghanistan would damage the Taliban "militarily and psychologically."

"This is going to be a gradual process," Levin, a Michigan Democrat, said on CNN's Larry King Live. "As we're tightening the noose, it seems to me a lot of important things can be done, including the gathering of the support of a number of opposition forces in Afghanistan."

Earlier in the day, officials confirmed that special forces were in northern and southern Afghanistan, searching for Taliban targets to strike and hunting for clues to the whereabouts of Osama bin Laden and his al-Qaida lieutenants.

The use of small numbers of U.S. troops in Afghanistan marked a shift to a broader range of military activities - overt and covert - that President Bush says is necessary to win the war.

A Pakistani military official said yesterday that American officials had informed his government that U.S. special forces would be conducting "hit-and-run" operations in Taliban-ruled areas of Afghanistan in a bid to flush out bin Laden, members of his al-Qaida network and Taliban leaders.

He said Pakistan was told U.S. forces have been in northern Afghanistan more than a week.

And Air Force special operations AC-130 gunships began attacking in southern Afghanistan. The high-firepower AC-130s typically give close air cover to forces on the ground or going in for small-unit operations.