KABUL/WASHINGTON -  The Taliban published an open letter to US President Donald Trump Tuesday warning him against sending more American forces to Afghanistan.

The statement called for the complete withdrawal of foreign forces from the war-torn country and comes as Trump weighs increasing the number of American troops there.

"Previous experiences have shown that sending more troops to Afghanistan will not result in anything other than further destruction of American military and economical might.

"Therefore it would be wise if you adopt the strategy of a complete withdrawal from Afghanistan instead of a troops increase," said the English-language letter, which was sent to the media.

US troops in Afghanistan now number about 8,400, a far cry from the US presence of more than 100,000 six years ago. They mainly serve as trainers and advisers to Afghan forces.

Trump's administration is currently finalising a new policy for Afghanistan almost 16 years after the US invasion to topple the Taliban regime.

American military commanders in Afghanistan have requested thousands of extra boots on the ground to help combat a resurgent Taliban.

Defense Secretary Jim Mattis is believed to be close to presenting the new US military strategy to Trump, with an announcement potentially coming in weeks.

The Taliban, which ruled Afghanistan from 1996-2001, also slammed the Afghan government in the letter and warned the United States that it could never defeat the insurgent group.

"It is entirely possible that you are being provided rosy pictures about Afghanistan by the stooges you have installed," it said. "You must realize that these repulsive sellouts neither care about your interests nor that of their own nation," the statement added.

The letter described foreign occupation as the "main driver of war" in Afghanistan and ended by making reference to reports that Trump may be considering outsourcing operations to private contractors.

Pentagon chief Jim Mattis said Monday he backs the top US commander in Afghanistan - after President Donald Trump reportedly suggested firing him.

NBC News this month said Trump had told Mattis and General Joe Dunford, the chairman of the Joint Chiefs of Staff, they should replace General John Nicholson, who heads up US and NATO forces in Afghanistan.

Nicholson "is our commander in the field. He has the confidence of NATO, he has the confidence of Afghanistan, he has the confidence of the United States," Mattis told Pentagon reporters.

Trump "is looking at all aspects of our effort over there, as he must in his responsibilities as the commander-in-chief," Mattis added. When asked if he personally backed Nicholson staying in the job, Mattis said: "Of course."

Trump has yet to announce a strategy for Afghanistan, but Mattis said one is "very, very close." Possible plans include sending thousands more troops into the nearly 16-year conflict, or taking the opposite tack and pulling out, leaving private military contractors to help the Afghans oversee the fragile security situation.