More than 500 Scottish-based Marines are to spearhead the next British deployment to Afghanistan in October, in their third operational tour of Helmand province in six years and the second since last April.

The men of 45 Commando, from HMS Condor, near Arbroath, will be sent as part of 3 Commando Brigade to replace the paratroopers and two Scots infantry battalions of 16 Air Assault Brigade fighting the Taliban.

About 400 Marines from 45 Commando also served as reinforcements for the invasion of Iraq in 2003, although because the entire unit did not go to war as a single body with its own headquarters, the Ministry of Defence does not count it as a full deployment.

In effect, however, the Arbroath-based Marines have carried out four testing combat operations in six years to relieve pressure on the overstretched and under-strength British Army.

The pending six-monthly tour will breach the MoD's own "harmony guidelines" which say that battalions should have 24 months between operational missions to allow troops to wind down, retrain, and spend time with their families.

The mountain and Arctic warfare specialist unit has lost four dead in Afghanistan and one in Iraq since 2001, and another two soldiers attached to it have also died.

The new force for Helmand will include 42 Commando, Royal Marines, as well as the 2nd battalion of the Royal Gurkha Rifles, the 1st battalion of the Rifles, and the 1st Queen's Dragoon Guards.

It will also include the first of the additional force elements announced by Defence Secretary Des Browne last month which will see the British fighting force in the country rise from 7800 to over 8000 by next spring.

A total of 110 soldiers have lost their lives in Afghanistan.

Meanwhile, Nato yesterday confirmed one of its soldiers was killed and four others wounded in a roadside bomb blast in eastern Afghanistan.

An alliance spokesman said the soldiers were on patrol in the eastern province of Kunar. It did not release the nationality of the dead soldier but most of the troops in the area are American.

The US also announced that an aircraft carrier is to be diverted from the Iraq war to the Gulf of Oman to add its striking power to coalition forces operating inside Afghanistan.

Defence officials said the transfer of the USS Abraham Lincoln would shorten the time that the carrier's pilots had to fly to support combat troops.

One official said the decision reflected the upsurge in Taliban activity in Afghanistan and improvements in security in Iraq. US Defence Secretary Robert Gates said last week he had "real concern" about a sharp rise in attacks in eastern Afghanistan, which he blamed on Pakistan's failure to put pressure on insurgents operating from Islamabad's side of the border.

The US Navy routinely moves ships in and out of the Gulf, where they not only support America's two current wars but also serve as a show of force to Iran.