Washington: The United States is "not winning" the war against Taliban insurgents in Afghanistan, US Defense Secretary Jim Mattis told Congress on Tuesday, promising to brief lawmakers on a new war strategy by mid-July that is widely expected to call for thousands more US troops.

The remarks were a blunt reminder of the gloom underscoring US military assessments of the war between the US-backed Afghan government and the Islamist militant group, classified by US commanders as a "stalemate" despite almost 16 years of fighting.

"We are not winning in Afghanistan right now. And we will correct this as soon as possible," Mr Mattis said in testimony to the Senate Armed Services Committee. Mr Mattis acknowledged that he believed the Taliban were "surging" at the moment, something he said he intended to address.

Some US officials questioned the benefit of sending more troops to Afghanistan because any politically palatable number would not be enough to turn the tide, much less create stability and security. To date, more than 2,300 Americans have been killed and more than 17,000 wounded since the war began in 2001.