The top US commander in Afghanistan has warned that the war there could be lost unless there is an increase in troops within a year.

In a leaked report, General Stanley McChrystal says a new strategy is needed and warns that "inadequate resources will likely result in failure".

The report, which was published on The Washington Post website, says that failure to reverse the momentum of the insurgents in the next 12 months could mean that victory is no longer possible.

While some parts of the report have been removed at the request of the United States Government, General McChrystal's spokesman in Kabul has confirmed it is a genuine copy.

He also says a key weakness of the international force is that it is not defending the Afghan population, and some of its tactics alienate civilians.

General McChrystal is expected to ask for a troop increase in the coming weeks as his forces fight to hold back a resurgent Taliban.

The report stresses the need to engage with the Afghan people using a "new strategy" that requires a "dramatically" different approach to the war.

"Inadequate resources will likely result in failure. However, without a new strategy, the mission should not be resourced," General McChrystal wrote.

He has already drawn up his request for more troops, which some officials expect will include roughly 30,000 new combat troops and trainers, but he has yet to submit it to Washington for consideration.

The Pentagon says it is discussing how he will submit it.

A request for more troops faces resistance from within US President Barack Obama's Democratic Party which controls Congress, and opinion polls show Americans are turning against the nearly eight-year-old war.

Mr Obama has said he wants to wait to determine the proper strategy for US forces in Afghanistan before considering whether more troops should be sent there.

"I just want to make sure that everybody understands that you don't make decisions about resources before you have the strategy ready," he said.

Deteriorating situation

In his assessment, Gen McChrystal painted a grim picture of the war, saying the overall situation was deteriorating.

He called for a revolutionary shift in strategy which puts as much emphasis on gaining the support of Afghans as it does on killing insurgents.

"Our objective must be the population," he wrote.

"The objective is the will of the people. Our conventional warfare culture is part of the problem, the Afghans must ultimately defeat the insurgency."

The war in Afghanistan is now at its deadliest in eight years.

General McChrystal's assessment said fighters had control over entire sections of the country, although it was difficult to say how much because of the limited presence of NATO troops.

He also strongly criticised the Afghan government as having lost the faith of the country's people.

"The weakness of state institutions, malign actions of power-brokers, widespread corruption and abuse of power by various officials, and ISAF's ( International Security Assistance Force) own errors, have given Afghans little reason to support their government," General McChrystal said.

General McChrystal's spokesman, Lieutenant-Colonel Tadd Sholtis, said that while the assessment made clear General McChrystal does not believe he can defeat Afghanistan's insurgency without additional troops, he could carry out a mission with different goals if ordered to by Mr Obama.

"The assessment is based on his understand of the mission as it was presented to him. If there's a change in strategy, then the resources piece changes," he said.

The number of US troops in Afghanistan has almost doubled this year from 32,000 to 62,000 and is expected to grow by another 6,000 by the year's end.

There are also some 40,000 troops from other nations, mainly NATO allies.

Fifty-eight per cent of Americans now oppose the Afghan war while 39 per cent support it, according to a recent CNN/Opinion Research poll.

Mr Obama's congressional critics, including his 2008 Republican presidential opponent Senator John McCain, have urged the administration to approve the deployment of more troops immediately, saying any delay puts the lives of troops already in Afghanistan at greater risk.

Senate Republican leader Mitch McConnell told CNN on Sunday his party would support a troop increase if needed, adding he was troubled by the delay in the decision-making.

"We think the time for decision is now," Senator McConnell said.

-ABC/Reuters