CAMP BASTION, Afghanistan - Gordon Brown made his first visit to Afghanistan since becoming prime minister on Monday and told troops the battle against the Taliban was vital to defeat terrorism.

Britain has some 7,700 troops in Afghanistan, the largest foreign force after the United States, and has borne the brunt of some of the fiercest fighting in the country as its soldiers combat Taliban insurgents in the southern province of Helmand.

Brown visited Camp Bastion, the biggest British base in Helmand, some 60 miles (100 km) away from where British and Afghan forces are engaged in a big offensive to seize the Taliban's most important stronghold, the town of Musa Qala.

"I want to thank you for what you have done in what is the front line against the Taliban," Brown told the troops. "This is one of the most challenging environments, it's one of the most difficult tasks, it is the most testing of times and it's one of the most important of missions.

"To win here and defeat the Taliban and to make sure we can give strength to the new democracy of Afghanistan is important in defeating terrorism all around the world," he told several hundred troops circled around him at the dusty desert base.

Brown, who replaced Tony Blair as prime minister in June, is due to make a statement to parliament on Wednesday setting out changes in policy towards Afghanistan expected to include a bigger stress on economic development.

Afghan army troops entered Musa Qala on Monday, the fourth day of a large offensive to recapture the town which began when mainly British troops surrounded it on Friday.

"I know this weekend in Musa Qala, some of you here and many of you who are up there have been doing a very important job in clearing the Taliban from that area," Brown told the troops.

"I believe if we can succeed there, as we will, and if we can work with the Afghan forces, then we can move forward events in Afghanistan in favour of a more peaceful future for this country."

Brown came to Afghanistan from Iraq where he announced that Britain would hand over responsibility for the last province it controls there to Iraqi security forces in the next few weeks.