Hundreds of U.S. troops pressed toward rebel fighters in rugged mountain caves Tuesday, while warplanes bombed dug-in enemy positions in the fiercest battle in Afghanistan in nearly a year.

At least 18 rebels were killed in the assault. The U.S. military believes the fighters are loyal to renegade warlord Gulbuddin Hekmatyar, a powerful Pashtun strongman who has vowed to link his forces with remnants of Al Qaeda and the ousted Taliban regime.

About 80 rebels were believed to be remaining in the southeastern cave network, under attack from 350 troops, including soldiers from the 82nd Airborne Division, U.S. Special Forces and allied Afghan militia.

"It's the largest concentration of enemy forces since Operation Anaconda," military spokesman Col. Roger King said, referring to a fierce eight-day battle in March against Taliban and Al Qaeda holdouts in a different area of southeastern Afghanistan, about 250 miles northeast of the current fighting.

While King said evidence pointed to Hekmatyar's military arm, the Hezb-e-Islami movement, he gave no further details, and a former high-ranking Taliban member questioned that.

Obeidullah, who goes by only one name, told The Associated Press that the fighting was being led by two ex-Taliban -- Hafiz Abdul Rahim, the regime's former chief of the border security, and Sirajuddin, former district chief of Shindand in western Afghanistan.

Obeidullah was assistant to the Taliban's intelligence chief.

The fighting, which started Monday, had ended by Tuesday evening, and coalition forces were searching the caves where the rebel forces had been hiding, said Lt. Col. Dave Lapan, a Pentagon spokesman. The coalition soldiers found a weapons cache at the site, Lapan said, adding he had no details on the specific number or type of arms found.

Lt. Col. Michael Shields, operations officer for the Coalition Task Force's 82nd Airborne Division, said there were indications that some of the entrances to the caves were camouflaged.

"The number of caves is far greater than we anticipated," Shields said, adding that American forces had been unaware of the cave network before fighting began.

The battle, about 15 miles north of Spinboldak, marked an important shift in the challenges facing the U.S.-led coalition with guerrilla forces massing for a protracted fight.

The U.S.-led forces received aerial support from American B-1 bombers, which dropped 19 2,000-pound bombs. F-16 fighters flown by unidentified European allies dropped a pair of 500-pound bombs, while AC-130 gunships and Apache AH-64 helicopters strafed enemy positions with rocket and cannon fire, King said.

The fighting was triggered by a small shootout pitting armed attackers against U.S. Special Forces and their Afghan government allies who were working to clear a mud-walled compound.

One attacker was killed, one injured and one detained, King said. The detained suspect told interrogators that a large group of armed men had massed in mountains nearby, King said.

Apache helicopters sent to investigate came under small arms fire, and then fighter aircraft went to pound the area, King said.

"We've had reports of various numbers of armed men, groups of people trying to gather in order to carry out attacks on the coalition," King said. "We've been actively engaged in trying to develop intelligence that would lead us to a precise location and yesterday [Monday] we did."

Hekmatyar was a key guerrilla commander during the 1980s Soviet war in Afghanistan. Later, in the civil war that paved the way for the Taliban takeover, Hekmatyar's men pounded the capital, Kabul, with daily rocket barrages. He lived in exile in Iran during the five years of the Taliban rule, and returned after U.S.-led forces ousted the hardline militia. Western intelligence agencies suspect he is getting money from Iran.

The CIA tried to kill Hekmatyar in May, firing a Hellfire missile at him from an unmanned Predator spy plane in an attack near Kabul. Several of his followers were killed.

Recently, reports have surfaced that Hekmatyar's men were among militants being trained in the Urgun mountains, about 215 miles northeast of the current fighting -- which would be the first pitting Hekmatyar's men against American troops in open battle.

Because the battle was near the border, it was possible that fighters of other nationalities were involved, King said.

Many Taliban and Al Qaeda suspects fled into Pakistan following U.S. bombardment in late 2001, and U.N. and American forces are concerned that Al Qaeda and Taliban militants are training again in southeastern Afghan mountains.

There have been a series of attacks along the border in recent months, including one in December that killed a U.S. army sergeant. Rockets frequently are fired at U.S. bases in that region but rarely hit their targets.

King wouldn't say whether U.S. troops might pursue rebels into Pakistan if they fled over the border, but said it shouldn't be necessary. Pakistan has said U.S. troops won't be permitted to pursue the enemy into its territory.