The comments came as Defence Secretary John Reid began a whirlwind tour of Afghanistan.

"We are discussing the various options that exist with NATO and our partners and we will make a decision in due course as to exact numbers when it is appropriate," a Ministry of Defence spokesman said.

The final figure of troops would likely be around 1,500 but it could be as many as 4,000, sources said. Britain already has 900 soldiers on the ground.

Reid outlined hopes to send a "sufficient" number of troops into the Helmand region next year to bolster efforts to seek out Al Qaeda-linked fighters and take on powerful warlords behind the world's largest heroin market.

The minister, who is on his first ever trip to Afghanistan, said the force would be in addition to a commitment to take control of Nato's Allied Rapid Reaction Corp (ARRC) next May.

Reid, who is due to hold talks with Afghan President Hamid Karzai and defence minister General Abdul Rahim Wardak to offer the increased British assistance, said he hoped to meet key objectives in Afghanistan over the next 12 months.

Speaking in Kabul, he was quoted by Britain's domestic Press Association as saying: "One of them is to develop ISAF's (the International Security Assistance Force's) presence in the south of Afghanistan, to supplement and extend the presence in the north and the west.

"But if we are going to do that I want to make sure it is of sufficient size to accomplish the task. It would be a sufficient number of soldiers, but would also need a degree of mobility."

He said the troops would be charged with counter-terrorism, counter-narcotics and counter-insurgency work.

The southern provinces account for the majority of Afghanistan's heroin production -- with Helmand responsible for half of all opium yields.

Officials admit those areas are beyond most control of authorities in Kabul and suspect the region may harbour Al Qaeda-linked extremists.

The United States -- which has troops in the south - has lost 51 soldiers this year, making it the bloodiest 12 months since the fall of the Taliban.

Reid admitted that a drive into Helmand could result in British fatalities, but said he thought the public would support the move.

He said Afghanistan had been a breeding ground for the extremists who attacked the United States on September 11, 2001.

"While we are in Afghanistan we are defending people in Britain, the US and the rest of the world by denying terrorists a base from which to plan their operations," the Press Association quoted Reid as saying.

"It is not a matter of casualties, but a matter of trying to avoid innocent casualties of terrorism."

At the same time, Reid denied any new push in Afghanistan would put a strain on resources focused on countering the increasing degree of resistance in Iraq.

"Whatever I need for Afghanistan is independent of the need to draw down on forces in Iraq. We will continue what we are doing in Iraq, until we achieve what we set out to achieve," he said.

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