The Pentagon has begun sending additional troops to Afghanistan to carry out United States President Donald Trump's new war strategy, and members of Congress will be briefed before details are made public, US Defense Secretary Jim Mattis said on Thursday.

“Yes, I've signed orders, but it's not complete,” Mattis told reporters in an impromptu news conference at the Pentagon.

He would not say how many additional troops are deploying or what their exact roles will be. He said that in general, the extra forces will support Afghan forces in combat with the Taliban, rather than doing the fighting for them.

Other officials have said the US will send about 3,900 additional troops. In a speech announcing his new strategy August 21, Trump did not mention that an increase in US troop levels was part of his new strategy.

He said “conditions on the ground, not arbitrary timetables,” will guide the strategy, and suggested troop levels will be kept largely secret.

“We will not talk about numbers of troops or our plans for further military activities,” Trump said.

On Wednesday, at Mattis's instruction, Pentagon officials said that about 11,000 US troops are in Afghanistan, acknowledging publicly for the first time that the 8,400 figure that officials had used for many months was inaccurate.

Mattis said he wanted to publicly clarify the current troop total before discussing how many more would be sent.

Mattis said he and other senior administration officials are scheduled to brief members of Congress on the latest deployments and the new war strategy next Wednesday. Critics have questioned whether sending a few thousand more troops will make a decisive difference in a war that began when US forces invaded to topple the Taliban regime in October 2001.

Officials have said the US plans to send as many as 3,900 more troops which would bring the number of publicly recognised troops in Afghanistan to about 15,000.

In the nearly 16 years since the United States went to war in Afghanistan, the number of American troops initially grew in spurts, as US leaders wavered about how much focus to put on the war.

President Barack Obama came into office in 2009 saying he would give the war there the attention it required, and the number of American troops on the ground spiralled by mid-2010 to 100,000.