The US has asked Britain to send more troops to Afghanistan in a move which military sources say could mean the deployment of an extra 1000-man battlegroup this year.

The request came during talks between Robert Gates, the newly-appointed US defence secretary, and Tony Blair at Downing Street on Sunday.

Meanwhile, in Afghanistan a Royal Marine from Arbroath-based 45 Commando died yesterday during a mission to oust Taliban insurgents in Helmand province, the MoD confirmed.

Nato officials said soldiers were attacking a militant base when they were "engaged from several insurgent positions".

He is the second British serviceman to die in the country in the past three days. Thomas Curry, a 21-year-old Royal Marine from east London, was killed during a battle to clear Taliban positions on Saturday.

Any new commitment of combat troops would further strain the already overstretched British Army, which is struggling to maintain rolling deployments of 5800 soldiers to Afghanistan and 7100 to Iraq every six months.

It is also stretching the RAF's ability to maintain two simultaneous "airbridges" to keep garrisons thousands of miles from the UK supplied. Sources say enough troops could be "scraped together" to provide the additional battlegroup requested by UK commanders in Helmand province last summer. But maintaining the commitment for anything beyond a few months would hinge on a "substantial" reduction of British force-levels in Iraq.

There are contingency plans to cut the Basra garrison by about 3000 men this summer, but these are dependent on improved security conditions in southern Iraq.

Senior officers also fear that a cutback will be impossible and that there might even be a need to reinforce the garrison if current US "surge" plans to pacify Baghdad trigger a countrywide Shi'ia militia uprising.

The Americans have 20,000 troops in Afghanistan and are about to take command of all Nato forces - including the British - next month. The Pentagon is desperate to reduce its commitment there to create reserves for Iraq and a possible confrontation with Iran.

UK commanders expect a renewed Taliban spring offensive in Helmand and neighbouring Kandahar from the end of February and would welcome more fighting troops and helicopters.

Fewer than 1000 of the existing force in Helmand are frontline soldiers, supported by just eight transport helicopters, to cover a volatile area four times the size of Wales. A senior military source told The Herald: "We can just about cope as things stand, but the toybox is almost empty as far as additional soldiers are concerned.

"While an extra battlegroup of 1000 to 1200 fighting troops would provide welcome tactical flexibility in Helmand this year, it could only be a stopgap measure unless we reduce the numbers in Iraq. Something has to give somewhere."

Those views are shared by experts in Helmand, where the Royal Marines have been leading a successful New Year offensive against the Taliban.

One source, who cannot be named for security reasons, told The Herald last week in Lashkar Gah, the Helmand capital, that the British need to beef up its presence in the lawless province.

"We need to bring in three more battalions and double the number of helicopters," the source said. "It's no secret that we have just eight Apaches, eight Chinooks and four or five Lynxs. On any given day, half of them will be out of action for servicing."

Britain faces the danger of a long stay in Helmand, with the mountainous north and desert south effectively in the hands of the Taliban, the source warned. Only extra troops, the source said, would enable the British to prise frontline villages from the hands of the extremists. British troops were initially only scheduled to stay in Helmand until 2009. Local troops, however, are not expected to be ready to replace Nato forces by then.

Last night Mr Gates arrived in Afghanistan where he plans to meet Afghan officials as well as US and NATO military commanders. Downing Street confirmed "strategic talks" had taken place with Mr Gates, but declined to comment on the content.

On his first visit to Nato headquarters in Brussels after replacing Donald Rumsfeld last month, Mr Gates said: "My first priority is making sure we preserve the gains we've made in Afghanistan and then talking about the way forward in Iraq."

He also accused Iran of doing nothing to help the US in Iraq. He said Washington could only engage with Tehran if it showed it was ready to be constructive, and the Iranians "clearly believe we're tied down in Iraq, that they have the initiative".

He added: "In addition, they have supported Hizbollah's efforts to create a new conflict in Lebanon," he told a news conference after talks with Nato Secretary-General Jaap de Hoop Scheffer.

"My view is that when the Iranians are prepared to play a constructive role in dealing with some of these problems, then there might be opportunities for engagement."