The Pentagon will send almost 4,000 additional American forces to Afghanistan, a Trump administration official has said, hoping to break a stalemate in a war that has now passed to a third U.S. commander in chief. The deployment will be the largest of American manpower under Donald Trump's young presidency.

The decision by Defence Secretary Jim Mattis could be announced as early as next week, the official said. It follows Trump's move to give Mattis the authority to set troop levels and seeks to address assertions by the top U.S. commander in Afghanistan that he doesn't have enough forces to help Afghanistan's army against a resurgent Taliban insurgency. The rising threat posed by Islamic State extremists, evidenced in a rash of deadly attacks in the capital city of Kabul, has only fuelled calls for a stronger U.S. presence, as have several recent American combat deaths.

The bulk of the additional troops will train and advise Afghan forces, according to the administration official, who wasn't authorised to discuss details of the decision publicly and spoke on condition of anonymity. A smaller number would be assigned to counter terror operations against the Taliban and IS, the official said.

Daulat Waziri, spokesman for Afghanistan's defence ministry was reluctant to comment on specifics but said the Afghan government supports the U.S. decision to send more troops. "The United States knows we are in the fight against terrorism, " he said. "We want to finish this war in Afghanistan with the help of the NATO alliance."

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US Army soldiers from the 2nd Platoon, B battery 2-8 field artillery, fire a howitzer artillery piece.Reuters/Baz Ratner

An Afghan lawmaker, Nasrullah Sadeqizada, however, was skeptical about additional troops and cautioned that the troop surge should be coordinated with the Afghan government and should not be done unilaterally by the United States. "The security situation continues to deteriorate in Afghanistan and the foreign troops who are here are not making it better," he said.

Although Trump has delegated authority for U.S. troop numbers in Afghanistan, the responsibility for America's wars and the men and women who fight in them rests on his shoulders. Trump has inherited America's longest conflict with no clear endpoint or a defined strategy for American success, though U.S. troop levels are far lower than they were under Presidents Barack Obama and George W. Bush. In 2009, Obama authorized a surge of 30,000 troops into Afghanistan, bringing the total there to more than 100,000, before drawing down over the rest of his presidency.

Trump has barely spoken about Afghanistan as a candidate or president, concentrating instead on crushing the Islamic State group in Syria and Iraq. His predecessors both had hoped to win the war. Bush scored a quick success, helping allied militant groups oust the Taliban shortly after the Sept. 11, 2001, attacks, before seeing the gains slip away as America's focus shifted to the Iraq war. In refocusing attention on Afghanistan, Obama eliminated much of the country's al-Qaida network and authorized the mission that killed Osama bin Laden, but failed to snuff out the Taliban's rebellion.