American troops were still fighting their way toward the crash site of a U.S. military helicopter Wednesday evening, more than 24 hours after the aircraft was shot down in eastern Afghanistan with 17 troops aboard, U.S. military officials said.

The fate of the crew and passengers remained unknown, while persistent efforts by U.S. ground troops, supported by bombardment from attack aircraft, to reach the site were thwarted by gunfire, stormy weather and the heavily forested, hilly terrain of Konar province, near the Pakistani border.

"We do hope and pray that everyone is safe," said Col. James Yonts, a U.S. military spokesman, speaking by telephone from Kabul, the capital. But military officials in Washington said no signs of life had been detected as rescue teams struggled to reach the craft.

The downed helicopter, a Special Operations variant of the CH-47 Chinook, was carrying a team of Navy SEALs to be implanted in the combat zone, according to a senior defense official in Washington.

Troops on a second helicopter nearby reported Tuesday that the craft was likely brought down by ground fire. Other military officials said it might have been downed by a grenade or crashed while evading fire as it approached a landing zone. It went down in a rugged area about 10,000 feet in elevation, officials said.

Further hampering the rescue effort was the loss of a Predator drone that had provided imagery of the crash area Wednesday, according to the senior official. That aircraft might also have been shot down, the official said.

Marine Gen. Peter Pace, vice chairman of the Joint Chiefs of Staff, told a Senate committee that the incident appeared "to be a shootdown of one of our Special Operations helicopters," probably by a rocket-propelled grenade. "Our hearts go out to their families," Pace said.

Yonts told the Associated Press that the helicopter was taking fire from the ground when it crashed. "It is a very strong and determined enemy," he said.

The incident followed a three-month escalation of armed clashes that have killed between 240 and 465 suspected anti-government fighters, 29 U.S. troops, 43 Afghan police officers and soldiers, and 125 civilians.

Taliban and al Qaeda forces were once believed to have been reduced to a marginal force in the country, and a peaceful presidential election was held in October. Since then, however, insurgents have stepped up attacks, apparently hoping to sabotage parliamentary elections scheduled for September.

U.S. officials said it was not clear if the insurgent forces in the crash were from the revived Taliban militia or included foreigners. A Taliban spokesman has asserted responsibility for the attack. Afghan officials have repeatedly complained of fighters slipping into the country from Pakistan.

U.S. military officials would not comment on the number of forces involved, but they said the helicopter was shuttling troops into the area as part of Operation Red Wing, a sweep aimed at capturing or killing al Qaeda militiamen.

Late Wednesday, Yonts said a "large, aggressive ground force" of American troops was moving toward the crash site in an effort to rescue any surviving troops. He said U.S. forces had been able to see but not approach the downed helicopter.

"I don't want you to think they are just out there all by themselves," he said. "We are fighting our way to that helicopter." He added that there was also "active air cover" in the area but that the high elevation and a storm had impeded the rescue mission.

Yonts said the Chinook had taken small-arms fire but that it remained unclear what caused the crash. "It could have been anything from hostile fire to they were maneuvering from hostile fire and struck a tree," he said.

The governor of Konar, Asadullah Wafa, said the area has been a haven for al Qaeda members. Fighters linked to the Taliban, as well as loyalists of Gulbuddin Hekmatyar, a former minister who is now a fugitive opponent of the U.S.-backed government in Kabul, are also reportedly operating in the territory.

Wafa said fighters rarely stayed overnight in the harsh terrain where the fighting took place. "They come in to attack, and then they rush back into Pakistan," he said. In recent weeks, he added, militia attacks have killed as many as 15 people.

The U.S. operation in Konar is part of a larger spring campaign by U.S. and Afghan forces to flush out fighters from hideouts in the east and south of Afghanistan. Despite heavy losses, the militiamen have shown themselves to be hardy, well-equipped fighters who can wage battles for hours. There are about 18,000 U.S. troops in the country.

Staff writer Bradley Graham in Washington contributed to this report.