SHAH-E-KOT, Afghanistan -- U.S. Marine helicopter gunships blasted cave entrances Wednesday in the rugged mountains, seeking to stop al-Qaida and Taliban fighters from escaping after U.S. and Afghan troops seized control of this valley.

Afghan commanders said many al-Qaida and Taliban fighters -- including their commander, Saif Rahman Mansour -- got away before Afghan troops overran three villages and a commanding ridgeline early Wednesday.

U.S. officials said they were holding about 20 prisoners who were being interrogated.

Pentagon officials had repeatedly said the only choice facing the enemy troops was to "surrender or die," although Afghan commanders had been prepared to allow them to leave.

A U.S. officer estimated that 500 al-Qaida fighters were killed in the 12-day offensive in eastern Afghanistan. But Afghan troops said they found only 25 bodies in the initial sweep of the area. Others may be buried in caves that collapsed during the bombing.

Leading the final assault were Afghan commanders Zia Lodin and Gul Haider, who had floated the idea of a negotiated exit.

"They're trying to slip away," one Afghan commander, Mohammed Qasim, said of the al-Qaida and Taliban fighters. "They're going in different directions in the mountains" toward Pakistan, he said.

In an attempt to block their escape, U.S. helicopter gunships, including Marine Cobras, blasted cave entrances Wednesday with rocket and machine gun fire.

About 500 Canadian troops landed high in the snow-capped mountains to search for al-Qaida fugitives. U.S. helicopters that brought them in flew close to the jagged terrain to avoid possible enemy fire. The Canadians, who were joined by about 100 U.S. troops, made their way up icy mountain trails, carefully avoiding unexploded ordnance littering the area after days of intense American airstrikes.

Lt. Col. David Gray, an operations officer of the U.S. 10th Mountain Division, said about 500 enemy fighters were killed in Operation Anaconda, mostly non-Afghans from Osama bin Laden's terrorist network. "What we have done is denied al-Qaida of its most important, well-trained fighters," he said.

However, it was unclear how many enemy fighters died in the biggest U.S.-led offensive of the five-month Afghan war, which brought conventional American ground troops into combat for the first time in the five-month conflict.

Initial U.S. estimates put the total number of enemy combatants at 150 to 200, but the figure later was increased to 500 to 600.