Senior military commanders told FOX News Wednesday that 17 soldiers were believed dead after their Chinook (search) helicopter crashed in eastern Afghanistan.

While the statement was not definitive, the military said they did not expect to find survivors at the crash site. The crash would be the deadliest blow yet to American forces in Afghanistan.

The officials said they knew of no communications from the crash area, accessible only by foot.

Rescue efforts have been stymied by inclement weather since Tuesday, when the MH-47 helicopter crashed. The special operations chopper was transporting reinforcements for troops already on the ground pursuing Al Qaeda (search) militants near the Pakistan border.

Officials cited reports from the region that the helicopter struck or landed badly on the side of a mountain before tumbling into a ravine — a scenario that suggested little hope of survival. They said, however, they could not confirm the deaths, and spoke on condition of anonymity since rescue operations were still underway.

The loss of the helicopter follows three months of unprecedented fighting that has killed about 465 suspected insurgents, 43 Afghan police and soldiers, 125 civilians, and 29 U.S. troops. Afghan and U.S. officials have predicted the situation will deteriorate before legislative elections are held in September.

The Taliban (search) have stepped up attacks, and there are disturbing signs that foreign fighters — including Al Qaeda — might be making a new push to sow mayhem. Afghan officials say the fighters have used the porous border with Pakistan to enter the country, and have called on the Pakistani government do more to stop them.

Even before the crash was announced, a Taliban spokesman claimed responsibility and said he had footage of the attack. As of Wednesday, no video had surfaced.

U.S. military spokesman Col. James Yonts said the helicopter was fired on as it was approaching a landing zone while rushing reinforcements to a battle in an area known to harbor "terrorist organizations." It flew on, but crashed about a little over a mile away at dusk, he said.

"The aircraft was taking indirect fire and direct fire from elements on the ground," he said.

Coalition and Afghan troops "quickly moved into position around the crash to block any enemy movement toward or away from the site," a U.S. military statement said. Yonts said fighting was continuing Wednesday.

Beside the bad weather, recovery operations were also hampered by the rugged terrain of the remote crash site, reachable only by foot, officials said. The crash took place in the mountains near Asadabad, in eastern Kunar province.

The helicopter was carrying forces into the area as part of Operation Red Wing against Al Qaeda militants.

Afghan Defense Minister Rahim Wardak told The Associated Press this month that intelligence indicates at least half a dozen Al Qaeda agents had slipped into the country recently, and that two of them blew themselves up in car bombs.

The downed chopper was carrying Navy SEALs (search), one U.S. official said. Another said it was carrying special operations forces but was unsure if they were SEALs or from another unit. The officials spoke from Washington on the condition of anonymity because rescue operations were still under way.

U.S. Vice Chairman of the Joint Chiefs of Staff Gen. Peter Pace said the "tragedy ... appears to be a shootdown of one of our special operations helicopters."

"We think it was a rocket-propelled grenade, sir, but not 100 percent sure. And that will come out in time as we're able to get to the scene and the investigation required," Pace told a Senate committee during a hearing on his nomination to be chairman of the joint chiefs.

"Our hearts go out to their families," Pace said.

Kunar Provincial Gov. Asadullah Wafa told the AP the Taliban downed the aircraft with a rocket. He gave no other details.

Purported Taliban spokesman Mullah Latif Hakimi telephoned the AP to claim responsibility for downing the chopper. He also claimed that rebels killed seven U.S. soldiers in an attack in the same area, although U.S. spokeswomen Lt. Cindy Moore said no such attack had been made on an American convoy.

Hakimi often calls news organizations to claim responsibility for attacks on behalf of the Taliban. His information has sometimes proven untrue or exaggerated, and his exact tie to the group's leadership is unclear.

The crash was the second of a Chinook helicopter in Afghanistan this year. On April 6, 15 U.S. service members and three American civilians were killed when their chopper went down in a sandstorm while returning to the main U.S. base at Bagram.

In some of the latest fighting, suspected rebels detonated a roadside bomb under a police vehicle in the same province as the helicopter crash, killing a district police chief and two other officers, said Zahar Murad, a defense ministry spokesman in Kabul.

On May 31, U.S.-led coalition soldiers gave the 8,000-strong NATO force responsibility for security in much of western Afghanistan. The International Security Assistance Force, currently under NATO command, already maintains security in the capital, Kabul, and other parts of the nation.

The transfer of authority was intended to free up troops in the 18,000-soldier U.S.-led coalition to concentrate on hunting Al Qaeda and Taliban holdouts in the south and east of the country.

The Associated Press contributed to this report.