A new Washington Post-ABC News poll indicate a majority of Americans believe the war in Afghanistan is not worth fighting, and most want fewer American soldiers to be sent there.

Fifty-one per cent of respondents said the war was not worth fighting, a six per cent increase in the negative column since last month.

And 45 per cent wanted to see troop levels reduced, a 16 per cent rise since January in those favouring troop reduction.

John McCain, the Republican senator who lost to Obama in last year's presidential election, said on Sunday that the "clock is ticking" on American public opinion of the Afghan war.

"I think you need to see a reversal of these very alarming and disturbing trends on attacks, casualties and areas of the country that the Taliban has increased control of," he told ABC's This Week programme, adding that there were not enough troops on the ground.

Possible troop increase

Mullen declined to comment on US media reports that the new commander of US and Nato forces in Afghanistan, General Stanley McChrystal, might recommend additional increases of up to 45,000 troops.

Reports suggest McChrystal may ask for more US troops to be deployed in Afghanistan [AFP]

But he left open the possibility, saying he would evaluate the need for more troops after reviewing a report by McChrystal.

"We'll see where that goes once the assessment is in here," Mullen said.

"And I've had this conversation with the president, who understands that whatever the mission is, it needs to be resourced correctly."

McChrystal's assessment report, originally due in mid-August, is now expected after the Afghan election process is completed.

Counting is under way following Thursday's presidential election, but there are concerns about the credibility of the vote after Abdullah Abdullah, the main challenger to incumbent Hamid Karzai, alleged vote rigging on Sunday.

A credible election result is important for the country and for Obama, who has made stabilising Afghanistan a top foreign policy priority.

Obama's plan

Obama already plans to increase the number of US troops in Afghanistan to about 68,000 by year's end, more than double the 32,000 the US had there before he took office at the beginning of the year.

Mullen said the US faced a multi-year effort to establish security and enable Afghan forces to maintain it.

"I don't see this as a mission of endless drift. I think we know what to do, we've learnt a lot of lessons from Iraq, focusing on the Afghan people," he said.

"I've said from a military perspective I believe we've got to start to turn this thing around from a security standpoint in the next 12 to 18 months.

"And I think after that we'd have a better view of how long it's going to take and what we need to do."