"There's got to be an exit strategy," Mr Obama said in an interview with the CBS "60 Minutes" programme on Sunday. "There's got to be a sense that this is not a perpetual drift."

Mr Obama's comments come as his administration prepares to roll out its new strategy for Afghanistan amid rising insurgent violence that has called into question the viability of a seven-year-old US-led effort to create a functioning democracy.


The US president announced last month that he would send 17,000 more US troops to Afghanistan this spring and summer in response to a deteriorating security situation, adding to the 36,000 already there.

"You know I think it is the right thing to do. But it's a weighty decision because we actually had to make the decision prior to the completion of (the) strategic review that we were conducting," he said.

US commanders have said that as many as 30,000 additional troops are needed to overcome a stalemate in parts of Afghanistan.

Mr Obama also signalled that the US was redefining its role in Afghanistan. While the Bush administration favoured a broad goal of promoting democracy, Mr Obama said his primary goal was simply to get Afghanistan to a point where it was not used as a site from which to launch attacks on the US. He narrowly defined the US mission in Afghanistan as: "making sure al-Qaeda cannot attack the US homeland and US interests and our allies. That's the number one priority."

"And in service of that priority there may be a whole host of things that we need to do," he said. "We may need to build up economic capacity in Afghanistan. We may need to improve our diplomatic efforts in Pakistan."

"We may need to bring a more regional diplomatic approach to bear. We may need to coordinate more effectively with our allies. But we can't lose sight of what our central mission is," he said.