US Marines poured into a lawless town in Afghanistan’s southern Helmand province — the first time a sizeable American force has fought in the largely British sector of the country for several years.

About 2,400 Marines, many veterans of the war in Iraq, were involved in the dawn assault on Garmser. Helicopters and armoured vehicles surrounded the town, known as a Taleban stronghold and big staging post for drug traffickers involved in the thriving opium trade in Helmand.

“We haven’t seen anybody who isn’t carrying a gun,” said Major Tom Clinton, the commander of US forces at Forward Operating Base Dwyer, a small British camp ten miles (15km) west of Garmser. “They are trying to figure out what we are doing. They are shooting at us, letting us know they are there.”

Captain Kelly Frushour, a Marine spokeswoman, said American forces had sealed off the town and entered some areas but that the operation was continuing. The Marines came under machinegun and rocket fire. They also recovered rockets and bomb-making equipment. There were no reports of casualties. It is unlikely that the Marines, whose deployment was announced weeks ago as part of a US-led build-up of troops, will have surprised any Taleban commanders. The size and speed of their attack, however, will serve notice to insurgents in Helmand that this is likely to be a long, hard-fought summer. Both sides expect the violence to increase sharply after the poppy harvest, Helmand’s main commodity, is completed in the coming days and weeks.

For two years British forces have been trying to drive Taleban forces out of Helmand and help to restore the authority of the Afghan Government. The 7,500 British troops are concentrated mainly in the centre and north of the province but have not had the manpower to tackle the south. The Marine operation coincided with fresh violence elsewhere in Afghanistan, where a suicide bomber killed 18 people, most of them police anti-narcotics officers involved in a poppy eradication operation near the eastern city of Jalalabad.

The district chief and two Australian journalists were believed to be among those wounded in the attack, which was claimed by a Taleban spokesman.

The bombing followed the failed assassination attempt against President Karzai on Sunday, when gunmen opened fire on an independence day parade, killing three people.

At a special sitting of the Afghan parliament yesterday, the country’s top three security chiefs were made to account for the failure to protect the parade. Amrullah Saleh, the head of intelligence, Abdur Rahim Wardak, the Defence Minister, and Zarar Ahmad Moqbel, the Interior Minister, lost no-confidence votes against them but the parliamentary majority was too small to force their removal from office.

Mr Saleh admitted that the security forces failed in their duties despite a tip-off last month that an attack was likely. He said that the three attackers had hidden in a three-storey hotel near the parade ground 36 hours before the attack and were killed in their hotel room.

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