US Defense Secretary James Mattis announced Monday that America and several allies have committed to boosting their troop numbers in Afghanistan, following an address to the nation by the US president.

Senior White House officials say President Donald Trump has already authorized Mattis to deploy up to 3,900 more troops to Afghanistan.

"I have directed the Chairman of the Joint Chiefs of Staff to make preparations to carry out the president's strategy," Mattis said in a statement issued after Trump's address on the 16-year conflict.

"I will be in consultation with the Secretary General of NATO and our allies -- several of which have also committed to increasing their troop numbers," Mattis said.