President Barack Obama has warned his Afghan counterpart Hamid Karzai that the US may pull all of its troops out of his country by the year's end.

Mr Obama conveyed the message in a phone call to Mr Karzai, who has refused to sign a security agreement.

The US insists this agreement must be in place before it commits to leaving some troops behind for counter-insurgent operations and training.

Barbara Plett Usher reports from Washington.