Carl Levin, the top Democrat on the committee, said the US should first focus on improving the capabilities of the Afghan military before committing any more US troops to the conflict.

'Providing the resources needed for the Afghan Army and Afghan police to become self-sufficient would demonstrate our commitment to the success of a mission that is in our national security interest while avoiding the risks associated with a larger US footprint,' he said.

'I believe these steps should be urgently implemented before we consider a further increase in US ground combat troops beyond what is already planned to be deployed by the end of the year.'

John McCain, the ranking Republican, disagreed, saying waiting to send more troops would repeat mistakes made in Iraq.

'If we await the day when the Afghan National Army is increased in size and capable of carrying out all of these operations fully on its own, it may well be too late,' McCain said.