WASHINGTON - US Defence Secretary Robert Gates said it could be difficult for the United States to add troops in Afghanistan beyond the 68,000 already approved, despite a top commander's call for a further 10,000 troops next year.

'It would be a hard sell. There's no question about it,' Mr Gates said in a CNN interview to be aired on Sunday, excerpts of which were released on Friday.

President Barack Obama has approved a plan to more than double the number of US forces in Afghanistan to 68,000 troops by the end of 2009, up from about 32,000 in late December.

There are also 32,000 troops there from other countries under Nato's International Security Assistance Force, or ISAF. The top US commander in Afghanistan, Army General David McKiernan, has requested an additional 10,000 US troops for 2010.

Pentagon press secretary Geoff Morrell confirmed that Mr Gates' comments in the CNN interview on Wednesday reflected his reservations about moving beyond the 68,000-troop level.

'Between the American military commitment and our coalition partners - the ISAF partners - we will have about 100,000 troops in Afghanistan. That's only about 10,000 shy of what the Russians had and I think we need to think about that,' the defence chief told CNN in the interview excerpts.

'My view is that it would be a far better investment to focus on building the strength of the Afghan army and the Afghan police, making sure that of the numbers of people we have there, there are adequate trainers so that we can accelerate the growth of those forces,' he said.

In testimony before the Senate Appropriations Committee this week, Gates said he planned to assess the performance of US troops in Afghanistan later this year or early next year before recommending any further deployments to Obama.

'We need to look very carefully at how our strategy is proceeding some months down the road before I would contemplate forwarding a recommendation,' he said before the panel.

The Pentagon has not yet received Gen McKiernan's request for additional forces from US Central Command, which oversees US operations in Afghanistan. -- REUTERS