Obama has called his most formidable military, political and national security aides to the secure Situation Room of the White House, to brainstorm the way forward as he mulls sending thousands more US troops into battle.

The president is under intense pressure to reinvigorate US strategy, faced with a strengthening Taliban insurgency and souring US public opinion on the eight-year war, but aides say he will not make a final decision for weeks.

Vice President Joe Biden, Secretary of State Hillary Clinton and Defence Secretary Robert Gates were the top ranking civilian officials expected for a bracing session on a war some supporters fear could swamp Obama's presidency.

War commander General Stanley McChrystal, who warned in a leaked report that the conflict could be lost within a year without more troops, was also due to take part, either in person or by video link-up, the White House said.

Other top military brass included were General David Petraeus who heads US central command, Admiral Mike Mullen, chairman of the Joint Chiefs of Staff and Director of National Intelligence Admiral Dennis Blair.

CIA chief Leon Panetta was also due to take part, along with Richard Holbrooke, the US envoy to Afghanistan and Pakistan, as well as the US ambassadors to Islamabad and Kabul.

Obama warned after meeting NATO Secretary-General Anders Fogh Rasmussen on Tuesday that the United States could not fight the battle in Afghanistan alone, in remarks apparently aimed at European partners.

"This is not a American battle, this is a NATO mission as well," Obama said.

McChrystal has reportedly requested up to 40 000 more US soldiers to fight the Taliban, but Obama is considering whether current tactics are the best way to defeat al-Qaeda in Afghanistan and Pakistan.

Some experts however doubt whether even that kind of deployment will be sufficient to subdue the insurgency. They advocate a narrower strategy of surgical strikes designed to squeeze al-Qaeda.

Obama's task in building political support for any troop increase is being complicated by the fraud-tainted Afghan presidential election and widespread mistrust in Washington over the government of President Hamid Karzai.

Some Republican critics have accused Obama of undue delay on framing a new Afghan strategy and called for him to approve any request for more troops submitted by the Pentagon.

But The White House has cautioned it will be "weeks" before the president makes up his mind on a new strategy, arguing he must decide on a war plan before assessing resources needed to carry it out.

"This isn't going to be finished in one meeting, it's not going to be finished in several meetings," said White House spokesperson Robert Gibbs.

A CNN Opinion Research poll this month showed record levels of opposition to the war, with 58 percent of respondents saying they opposed it, while 39 percent were in favour.

Other recent polls have shown public opinion more evenly split on the war launched following the 11 September attacks in 2001.

Turning US sentiment on the conflict has coincided with a spike in deaths of US and allied troops.

More than 370 foreign soldiers have died in Afghanistan this year, as the Taliban has increased the use of deadly improvised explosive devices (IEDs).

The US military has declined to reveal the details of McChrystal's troop request but Republican Senator John McCain said in a weekend television interview that the commander had appealed for 30 000 to 40 000 forces.

Gates has said he will only formally convey McChrystal's request to Obama once the policy review is complete ? and denied any rifts between the Pentagon and some sceptics of troop increases in the White House.