The US army general, David Petraeus, made clear today that he would not be bound by Barack Obama's promise to start withdrawing US troops from Afghanistan by July next year.

In his first interview since taking command of Nato troops in the country less than two months ago, the top general said he could foresee circumstances where conditions would make it impossible for him to recommend a reduction in forces.

Such an outcome would scupper President Obama's plans not to appear bogged down in a worsening Afghanistan conflict as he prepares for re-election in 2012.

Speaking on US television from his headquarters in Kabul, Petraeus said the controversial promise made by Obama in November that a surge of 30,000 US troops would be temporary and would start to be reversed in July 2011 was simply an attempt to increase the urgency of the international effort in Afghanistan.

"I think the president has been quite clear in explaining that it's a process, not an event, and that it's conditions-based," Petraeus said.

Although seen at the time as a smart way of appeasing critics of the war within Obama's own party, the drawdown announcement has been widely criticised for emboldening insurgent groups who saw it as a sign of weakening resolve.

When asked by the Meet the Press programme whether he might reach a point where despite knowing the process of withdrawal is supposed to start he would nonetheless advise that it was impossible, Petraeus said: "Certainly yeah, again, the president and I sat down in the Oval Office and he expressed very clearly that what he wants from me is my best professional military advice."

One published account of Obama's discussions on the drawdown date with Petraeus and his other top generals paints a very different picture.

According to US journalist Jonathan Alter, whose book The Promise gives an account of the Afghan war led by Obama in November last year, the US president went to great efforts not to be "boxed in" by the US military establishment that supported long-term engagement in Afghanistan.

After a period of silence on taking command from his disgraced predecessor Stanley McChrystal, Petraeus is launching a media blitz to drum up support for the war, which is currently at an all-time low in the US. According to one public opinion poll seven out of 10 Americans do not believe the war can end successfully.

The death toll of US troops hit a record 66 in July, while the total number of western soldiers killed since the conflict began in late 2001 surpassed 2,000 today.

Petraeus was upbeat about the war's progress, saying Taliban foot soldiers were increasingly cut off from the insurgent leadership in Pakistan and that the Afghan government was making progress on rooting out corrupt officials.