Taleban fighters in the strategic Afghan town of Musa Qala in Helmand province say they are under attack from British and Afghan troops.There was no immediate confirmation of the attack from UK sources, but the governor of Helmand province said earlier that the assault was under way. The Taleban defences include hundreds of mines.
Musa Qala has been the main centre of drugs trading in Afghanistan since the Taleban retook the town in February.
A local Taleban commander said the Friday's attack began in the afternoon, from three directions. He said that British troops and Afghan army soldiers were moving together on the ground and there had been exchanges of fire.
The BBC's David Loyn in Kabul says this is the first significant move since an operation intended to take Musa Qala began on Tuesday. He adds it is the first big test of the new Afghan army, which is to take the lead in the offensive.
Hundreds of local people fled after tribal elders were told of the attack and warning leaflets dropped from the air.
The Taleban say they have more than 2,000 fighters ready for the defence of Musa Qala, the only substantial town they hold. For now, they say they are holding their ground.
The Taleban takeover of Musa Qala was in contravention of a deal brokered with tribal elders when British troops withdrew from the town last year. The highly controversial move was portrayed at the time as a "win-win" situation, with the Taleban and British pull-out leaving local forces to assume control of local government.
British officers maintained it was a redeployment rather than a withdrawal; freeing them up to take on Taleban forces in other parts of Helmand. It was even suggested that it could act as a model for reducing violence in the volatile province. But the deal was strongly criticised by some Afghan and US officials.