Afghanistan's relatively calm election was a welcome boost for President Barack Obama's strategy but with waning US public support for the war both countries will need quick military and political successes.

Experts say no matter who wins — with incumbent President Hamid Karzai the favourite — the ultimate victor's approach and whether Thursday's election is deemed credible will have a big impact on the success or failure of US strategy.

"We are going to need an understanding from whoever the new president is that Afghanistan is going to rise to the occasion," said Bruce Riedel of the Brookings Institution think tank.

"We have now put roughly 70,000 American soldiers into this war, we are committing billions of dollars in new assistance. We are living up to our end of the deal to resource the war properly," added Riedel, who oversaw the Obama administration's policy review this year of Afghanistan/Pakistan policy.

Zalmay Khalilzad, the former US ambassador to Afghanistan, said if Karzai wins Washington would have to improve relations with him and counter the incumbent's view that the United States had wanted someone else elected.

"What they (Obama administration officials) need to do is to establish a relationship of mutual trust. There have been some difficulties," said Afghan-born Khalilzad, who knows Karzai well.

There needs to be concrete proof early on that a new government is serious about good governance, fighting corruption and embracing the rule of law, said Alex Thier of the US Institute of Peace nonpartisan think tank funded by the US Congress.

"A failure to do that, particularly in Karzai's case, will lead to the further erosion of his legitimacy and also that of the government — and by extension the international efforts in Afghanistan," Thier said.

US special envoy to Afghanistan and Pakistan, Richard Holbrooke told reporters that after the vote count, for the United States corruption is "the most important issue".

Just as the new Afghan government faces pressure to improve its performance, so will Obama be forced to come up with a more defined approach to the war.

"(Obama) has provided a change of leadership, he's provided a strategy going from counterterrorism to counterinsurgency, and he's provided more resources," said Lawrence Korb of the Center for American Progress think tank.

The US Congress will want clear benchmarks to measure whether Washington's approach is working or whether it will become Obama's Vietnam War.

The Afghan election was a test for Obama's strategy aimed at reversing Taliban gains — and the fact that militants did not launch wide-scale attacks should be seen as a success for this new approach, said Riedel.

Nobody is predicting immediate military success, with Defence Secretary Robert Gates talking about an 18 to 24 month time frame for judging the new Obama strategy.

Afghans are not the only ones questioning the war, a string of recent US public opinion polls show.

A poll of published in the Washington Post on Thursday found 51 percent said the Afghan war was "not worth fighting", compared to 47 percent who said it was. Among Democrats, it was 70 percent to 27 percent against the war.

Riedel called the poll a "wake-up call" showing that "war weariness has set in, in the United States, and it has set in particularly in the president's own party". – Associated Press