KABUL, Afghanistan — Afghan and international forces on Monday retook a southern town held by Taliban militants since February, although foreign fighters -- possibly members of al Qaeda -- were continuing to attack invading troops, officials said.

Defense Ministry spokesman Gen. Mohammad Zahir Azimi said Afghan, British and U.S. forces had "completely captured" Musa Qala, a town in the opium poppy growing belt of northern Helmand province. A Taliban spokesman said its forces had retreated.

NATO's International Security Assistance Force said ISAF and Afghan troops had entered the outskirts of the main part of Musa Qala but would now proceed cautiously into the town center because of improvised explosive devices.

Afghan army commander Brig. Gen. Gul Agha Naebi said both sides were still exchanging fire. He said he thinks foreign fighters or al Qaeda members are putting up the remaining resistance.

"The town is surrounded," he said, adding that his troops are about 500 yards away from the Musa Qala market, in the center of the town. "The bombing continues. The area is big."

He said the troops would clear the streets of mines Tuesday.

Afghan and international troops have stepped up operations around Musa Qala since early November, and fighting in the area has intensified in the last several days as forces advanced on the town.

A Taliban spokesman, Qari Yousef Ahmadi, said militant fighters left Musa Qala as a strategic decision to avoid Taliban and civilian casualties. "Because of the massive bombings this morning, the Taliban didn't want to cause more casualties, so this afternoon all the Taliban left Musa Qala," Ahmadi told The Associated Press by satellite phone.

British Prime Minister Gordon Brown, visiting British troops, said he had "no doubt" the Musa Qala operation would be successful and that social and economic progress would follow military action.

"In Musa Qala the action has been taken, and I think we will see in the next few days in Musa Qala that the action will be effective, that it will work and it will bring long-term and lasting results," Brown said.

He traveled to the main British military base in southern Afghanistan on Monday to boost the spirits of troops serving on the front line of an increasingly deadly fight against Taliban militants.

Brown, at a news conference in Kabul with Afghan President Hamid Karzai, said he had "no doubt" the Musa Qala operation would be successful and that social and economic progress would follow military action.

A resident of Musa Qala, Haji Mohammad Rauf, said he saw Taliban fighters leave the town in trucks and motorbikes around noon. Two hours later, hundreds of Afghan soldiers streamed into town and established security checkpoints, he said.

"I was standing on my roof and saw hundreds of Afghan soldiers drive into town," Rauf said. "All the shops are closed and families are staying inside their homes."

A British military spokesman, Lt. Col. Richard Eaton, said he couldn't confirm that the Taliban had left the town's center but said he wouldn't be surprised.

"This is what happens. We have had a number of operations in the past where once the Taliban realize they are overmatched they tend to leave," Eaton said. "I wouldn't be surprised if that is the case here. Ultimately our aim is to take Musa Qala and if we take Musa Qala without a big fight, that's fantastic."

Taliban militants overran Musa Qala in February, four months after British troops left the town following a contentious peace agreement that gave security responsibilities to Afghan elders.

U.S.-led coalition troops carried out airstrikes Sunday against compounds used by Taliban weapons smugglers in Musa Qala, the coalition said Monday. Several militants were killed and two civilians were wounded, it said.

Following the airstrike, the joint Afghan and coalition forces came under attack as they searched compounds in the area. "Using a combination of accurate, conventional munitions and small arms, the combined force returned fire, killing the militants," it said. Ten suspects were detained.

Musa Qala is in the north of Helmand province, the world's largest opium poppy growing region -- and the front line of Afghanistan's bloodiest fighting this year.

Elsewhere, an Afghan army helicopter crashed in central Afghanistan on Monday because of bad weather, killing four people, the Defense Ministry said. The Mi-17 helicopter went down in Salar district of Wardak province, where the weather was foggy.

Two helicopters were traveling from Kabul toward western Afghanistan when one of them crashed, said Wardak provincial police chief Zafaruddin, who goes by only one name. Authorities recovered three bodies from the burning wreckage, he said.

In neighboring Sangin district, police clashed Monday with a group of Taliban militants, leaving 15 militants dead and 11 others wounded, said district police chief Mohammad Ali.

Authorities recovered some of the dead militants' bodies, Ali said. There were no casualties among Afghan troops, he said.

This year has been the deadliest since the U.S.-led invasion in 2001. More than 6,200 people have been killed in insurgency-related violence, according to an AP tally of figures from Western and Afghan officials.