International and Afghan troops have begun a major offensive to retake a strategic town in southern Afghanistan from the Taleban, Nato sources said.
Afghan troops are playing a key part in the operation
Nato says Afghan and British ground troops used heavy gunfire against the Taleban on the outskirts of Musa Qala.
US soldiers were dropped by helicopter before sunset to carry out an overnight assault using night vision equipment.
The Taleban say they have 2,000 troops defending the town in Helmand province, which they seized in February.
Musa Qala is the only substantial town the Taleban hold in Afghanistan. For now, they say they are holding their ground.
The BBC's David Loyn, in Kabul, says the town has been the main centre of drugs trading in the country since the Taleban moved in in February and has taken on symbolic importance for both sides.
'First significant move'
Friday's attack by British and Afghan forces began in the afternoon, from three directions. Further raids are expected to be carried out on Saturday.
The main assault is being conducted overnight by US troops.
The Taleban defences include hundreds of mines.
The Taleban rejected an offer from the governor of the province to lay down their arms, and late on Friday, he told the BBC that the problem of Musa Qala was "being sorted".
Hundreds of local people fled after tribal elders were told of the attack and warning leaflets dropped from the air.
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.