WASHINGTON - US Defense Secretary James Mattis said Thursday that he has signed orders to send additional troops to Afghanistan.

Mattis told reporters at the Pentagon that he will not reveal details until he briefs Congress next week.

However, he said that the additional troops will include advisers and support personnel that will enable the Afghan force to fight more effectively.

US media have reported that about 4,000 more troops will be deployed in Afghanistan.

The move came after US President Donald Trump unveiled his new Afghanistan strategy Monday night in a national address, calling a rapid exit of the US troops from Afghanistan "unacceptable" and pledging a shift from a time-based approach to one based on conditions.

On Wednesday, Pentagon spokeswoman Dana White said that 11,000 US troops are currently in Afghanistan, thousands more than the previously acknowledged 8,400.

The 8,400 number did not include troops on temporary assignment, according to the Pentagon.