'US reached decision on Afghan strategy after rigorous debate'

Mattis refused to hint at any details of the decision, which came after months of speculation over whether Trump, frustrated with a stalemate after 16 years in Afghanistan, would allow the Pentagon to boost troop numbers on the ground in the country.

However Mattis appeared satisfied after what he described as an in-depth review of the policy by much of the president's cabinet and top security officials at Camp David on Friday. "I'm very comfortable that the strategic process was sufficiently rigorous, and did not go in with a preset condition in terms of what questions could be asked and what decisions could be made," he said.

"Everyone who had equity was heard," he said, including budget officials responsible for funding the effort.

Trump had several options on the table, that ranged from backing away from the country to stepping up US efforts to defeat the Taliban.

In June he gave Mattis the power to increase troop numbers above the estimated 8,400 that have been in the country - close to 4,000 more, according to reports.

But Mattis said he was loathe to move before he had a true picture of the numbers, which he said were actually higher than 8,400, and before Trump had his say on the broader strategy.

"The president had to make strategic decisions," Mattis said.

"He delegated to me, when he came in, the tactical and operational decision. He did not delegate one ounce of the strategic decision."