SPIN BOLDAK, Afghanistan (Reuters) - U.S. and Afghan forces are fighting a large band of rebels in the mountains of southern Afghanistan in the biggest battle in the country for nearly a year, officials said Tuesday.

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

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 succeeded in wiping 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.

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

King said U.S. and allied warplanes had dropped 19 2,000-lb bombs and two guided 500-lb bombs on enemy positions near a series of caves in the remote hills.

"There were repeated gun engagements by the AC-130 and the AH-64 Apaches," he added.

In the southern border town of Spin Boldak, residents stood on rooftops and shipping containers to watch flames and smoke rising from the Adi Ghar mountain, 23 km (14 miles) to the north.

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

King said U.S. forces were still trying to establish how many rebels were in the area, although initial information had suggested it was a group of around 80 men.

"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 reinforcements had been sent Tuesday, with 300-350 mainly U.S. and some Afghan militia forces now involved.

Asked how long the operation might last, King said: "It's a relatively large area... It is rough terrain, it could take a considerable period of time."

Afghan government officials in Spin Boldak said 22 rebels had been killed and 11 captured, with 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 a rebel on Monday after coming under small arms fire close to Spin Boldak, who then told them of a larger group hiding in the mountains.

When Apache helicopters went to verify the information, 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, although most of those captured so far were "apparently Taliban."

Other officials said a former Taliban police chief of Kandahar, Hafiz Abdul Majeed, was believed to be leading the rebels, along with another Taliban commander Hafiz Abdur Raheem.

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 also played a leading role in the destruction of Kabul in the 1990s when warlords fought each other for control of the capital.