He was speaking after Nato talks in Brussels on June 11 amid growing unease over the mission's prospects.

Nato head Anders Fogh Rasmussen said it was still realistic to aim to start handing over security responsibilities to Afghan forces this year.

Nato officials concede there is likely to be increased violence over a long hot summer in Afghanistan as the coalition pursues a key operation in and around the southern city and province of Kandahar.

Mr Gates told reporters that Nato was making progress "slowly, but steadily and sustainably".

"No-one would deny that the signs of progress are tentative at this point, that they are almost anecdotal," he said.

"But if you talk to the people that have been there a while, their view is that the situation is slowly beginning to improve, and that we are recapturing the initiative."