KANDAHAR, Afghanistan, Mar 25 (Reuters) US forces were involved in heavy fighting with Taliban guerrillas in Afghanistan today but there was no immediate word on casualties, a provincial official said.

Afghanistan has seen a surge in attacks by Taliban insurgents and their militant allies in recent months and the Taliban have vowed to launch a spring offensive against US-led foreign forces and the Western-backed government.

The clash erupted after US troops backed by helicopter gunships launched an operation in the Sangin district of the southern province of Helmand, said an administrative official in the province.

''It's very heavy,'' the official, who declined to be identified, said of the clash.

Afghan forces had been sent to join the US forces battling the insurgents, he said. US and Afghan forces fought the biggest battle in months against Taliban fighters in the same district at the beginning of February.

A US military spokesman said he had no information about a clash while Taliban spokesmen could not be reached for comment.

Helmand has been a bastion of Taliban insurgents since US and Afghan opposition forces ousted their government in late 2001.

The province is also Afghanistan's main opium-growing region and the insurgents are in league with drug gangs, complicating efforts to bring security and stamp out drugs, security officials say.

British troops have been arriving in the province in recent weeks as part of an expansion of a NATO-led peacekeeping force into the Afghan south. In all, 3,300 British troops will soon be based in Helmand.

A British military spokesman said he had no word on British forces involved in fighting on Saturday.

REUTERS CH PM1202