US Defence Secretary Robert Gates arrived Thursday in Afghanistan, where the United States is looking to increase its military presence to fight a mounting insurgency.

Gates, who was to meet with commanders on the ground battling Taliban militants, said Washington wanted to get thousands more troops into Afghanistan but warned of the difficulties foreign forces have had in the country.

"We are going to try and get two additional brigade combat teams into Afghanistan by summertime," he told reporters on his plane. "How long it will be before we can get the others, we don''t know yet."

The two brigades would be in addition to a brigade from the 10th Mountain Division that is already scheduled to deploy in January.

That is still short of the more than 20,000 additional troops sought by General David McKiernan, the commander of both US and NATO forces in Afghanistan. There are currently 32,000 US troops in Afghanistan.

The general, who took Gates into a lunch with Dutch and British commanders in the troubled southern sector of Afghanistan, has asked for four additional combat brigades, an aviation brigade and other support troops.

But Gates said the incoming administration of Barack Obama should be careful in undertaking a build-up of foreign troops in a country that has often proved to be their undoing.

"The history of foreign military forces in Afghanistan, when they have been regarded by the Afghans as there for their own interests and as occupiers, has not been a happy one," he said.

"The Soviets couldn''t win in Afghanistan with 120,000 troops and they clearly didn''t care about civilian casualties. So I think we have to think about the longer term in this," he said.

"Making sure the Afghans are out in front is a key element, but also figuring out how many foreign troops is too many in terms of being successful," Gates said. "I think that is still an unanswered question."