US Defence Secretary Jim Mattis said he had signed deployment orders for some of the American reinforcements that are to be sent to Afghanistan.

It is the first tangible step that the Pentagon has taken to carry out the new strategy that President Trump announced last week to step up the fight in Afghanistan and try to take back the initiative from the Taliban, The New York Times reported.

"I have signed orders, but it is not complete," Mattis said as quoted by the paper. "In other words, I have signed some of the troops that will go and we are identifying the specific ones."

Although Mattis did not say which forces were being deployed, administration officials have previously said the Pentagon will send nearly 4,000 additional troops, some of whom are expected to come from the 82nd Airborne Division.

The US currently has about 11,000 troops in Afghanistan to advise Afghan forces and carry out counterterrorism missions.