The President-Elect's intention to shift the focus of the fight against terrorism to Afghanistan has been bolstered by Robert Gates agreement to stay on as Defence Secretary.

Mr Gates is a strong believer in an Afghan surge, which would not only put thousands more boots on the ground but involve negotiations with malleable branches of the Taliban.

It would also aim to boost co-operation with Iran and Pakistan where some elements have supported the anti-Western insurgency.

The need for more US troops in addition to the 32,000 already serving, has been accelerated by the Afghan presidential election in September 2009, and the voter registration process that begins in the New Year, Mr Gates said.

"The most important objective for us for 2009 in Afghanistan is a successful election," he said at a meeting of defence ministers from the eight countries fighting in southern Afghanistan. "One of the things we talked about was trying to surge as many forces as we can prior to the election, to try and provide a secure environment for the election."

The Pentagon chief, who has been in the position for two years, added that he wanted to send another three brigades of combat ground forces and an aviation brigade, beginning as early as next spring.

A Pentagon official said the plans are likely to be drawn up before Mr Obama takes office on Jan 20. Most will be sent to the poppy-growing South, where the need for more Western forces is greatest, and where 8,000 British troops are currently fighting.

A spokesman said the final number was likely to be "well north of 20,000", and indicated that countries such as Britain already fighting in the south would not face strong US demands to provide more troops.

Those would be placed on Nato participants not yet involved at the sharp end.

Defence department staff members are privately delighted Mr Obama has chosen to retain a popular boss in the interests of stability. Mr Gates is expected to be officially introduced as part of an experience and centrist national security team on Monday.

"The president-elect has already indicated that he wants to put additional troops in Afghanistan," said an official, who spoke on condition of anonymity.

Over the weekend Mr Obama reiterated that he wanted to begin drawing down the 146,000 US troops in Iraq as soon as he takes office, saying he wanted to "shore up efforts" in Afghanistan.

During the election campaign, Obama said his administration would dispatch two or three additional brigades to Afghanistan in 2009, on top of the brigade President George W Bush decided to send this January.

Each brigade contains between 3,500 and 4,000 soldiers.

Some estimates for the numbers needed to fight the Taliban are however now running much higher than the initial 20,000 foreseen by Mr Gates.

Current and former US officials have warned that a surge of forces into Afghanistan will not necessarily meet with the same success as the troop build-up in Iraq.

"Additional troops in Afghanistan may be necessary but they will not, by themselves, be sufficient to lead to the results we saw in Iraq. A similar confluence of events that contributed to success in Iraq does not appear to exist in Afghanistan," former defence secretary Donald Rumsfeld said in a recent newspaper opinion piece.

Some analysts believe Washington ultimately will need more than 100,000 troops to stabilise Afghanistan before the Afghan army is ready to take over security.

"I suspect that to succeed in Afghanistan, we're eventually going to have to swing a sizeable fraction of what we now have in Iraq into Afghanistan," said Stephen Biddle of the Council on Foreign Relations.

"The scale of the shift will be large, and the time needed to pull it off will be long," he said.