SPIN BOLDAK, Afghanistan (Reuters) - U.S. and Afghan forces were hunting remnants of a large band of rebels in mountain caves in southern Afghanistan Tuesday after the biggest battle in the country for nearly a year, officials said.

U.S. military spokesman Colonel Roger King said at least 18 rebels loyal to renegade commander Gulbuddin Hekmatyar had been killed in the fighting, and said it was the largest enemy force encountered since "Operation Anaconda" last March.

Officials said U.S. and Allied warplanes had pounded a cave complex with 2,000- and 500-pound bombs overnight Monday, and two U.S. rifle companies had been searching the area for survivors since Tuesday morning, without success.

"The number of caves is far greater than we anticipated," said Lt. Col. Mike Shields. "It's rugged terrain, it's slow, it's not something we are going to get through rapidly."

Shields said he had seen no evidence the rebels had escaped and the search was continuing. "As far as we are concerned it's not over and we are not going to treat it that way until we have a better assessment on the ground," he said.

As Washington shifts its attention to Iraq, the U.S. military has increasingly come under attack in Afghanistan, suggesting that 14 months after toppling the Taliban they have not wiped out the fundamentalist militia and their allies.

King said U.S. B-1 bombers, AC-130 gunships and Apache helicopters as well as Allied F-16 warplanes had attacked enemy positions for more than 12 hours throughout the night in a range of hills not far from the Pakistani border.

Norway's government said its warplanes had dropped bombs in battle for the first time since World War II.

In Anaconda last March, the biggest U.S. ground offensive of the Afghan war to date, about 1,500 U.S. troops tried to flush out a force of about 1,000 Taliban and al Qaeda militants from cave hideouts in eastern Afghanistan.

In the southern border town of Spin Boldak, residents stood on rooftops and shipping containers Tuesday to watch flames and smoke rising from the Adi Ghar mountain, 14 miles to the north.

King said U.S. forces were still trying to establish how many rebels were in the area, after initial information had suggested there were about 80 men in the group.

"It's without a doubt the largest concentration of enemy forces that we've come across since Operation Anaconda," King told reporters at Bagram air base, the U.S. army's headquarters in Afghanistan.

King said 300-350 troops, mainly U.S. and some Afghan militiamen, had been involved in the fighting.

Afghan government officials in Spin Boldak said 22 rebels had been killed and 11 captured, and a huge arms cache found in the mountains. They said at least one Afghan soldier had been killed, while King said there had been no coalition casualties.

RENEGADE WARLORD

King said a U.S. special forces team had captured one rebel Monday after coming under small-arms fire near Spin Boldak, who then told them of a larger group hiding in the mountains.

When Apache helicopters went to check, they were fired upon and called in ground and air support.

King said U.S. intelligence suggested the fighters were loyal to former Afghan Prime Minister and renegade warlord Hekmatyar. "We've had reports over the past several months that he has been attempting to consolidate with remnants of al Qaeda and Taliban," King said.

Khalid Pashtun, spokesman for the governor of the southern city of Kandahar, also said Hekmatyar's men were fighting alongside Taliban and al Qaeda fighters, though most of those captured so far were "apparently Taliban."

Afghan officials say the Taliban are trying to regroup in southern and eastern Afghanistan, and there have been regular small-scale attacks on U.S. and government positions recently.

Hekmatyar was a leading commander of the mujahideen militia that battled Soviet rule in Afghanistan in the 1980s, with considerable support from Pakistan and the United States. He played a leading role in the destruction of Kabul in the 1990s when warlords fought each other for control of the capital.