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In his first formal address to the nation as commander-in-chief, Trump discarded his previous criticism of the 16-year-old war as a waste of time and money, admitting things looked different from "behind the desk in the Oval Office".

"My instinct was to pull out," Trump said as he spoke of his frustration with a war that has killed thousands of US troops and cost US taxpayers trillions of dollars.

But following months of deliberation, Trump said he had concluded "the consequences of a rapid exit are both predictable and unacceptable" leaving a "vacuum" that terrorists "would instantly fill".

While Trump refused to offer detailed troop numbers, senior White House officials said he had already authorised his defence secretary to deploy up to 3,900 more troops to Afghanistan.

He warned that the approach would now be more pragmatic than idealistic. Security assistance to Afghanistan was "not a blank check" he said, warning he would not send the military to "construct democracies in faraway lands or create democracies in our own image".

"We are not nation building again. We are killing terrorists."

Afghanistan on Tuesday welcomed Trump's decision to renew US engagement in the war-torn country. 

President Ashraf Ghani said in a statement that the announcement showed the "enduring commitment by Afghanistan's foundational partner in this global conflict". 

Also on Tuesday, NATO chief Jens Stoltenberg welcomed Trump's plan, saying the alliance would not let the country become a haven for terrorists.

"Our aim remains to ensure that Afghanistan never again becomes a safe haven for terrorists who would attack our own countries," he said in a statement.

The US has grown increasingly weary of the conflict that began in October 2001 as a hunt for the 9/11 attackers has turned into a vexed effort to keep Afghanistan's divided and corruption-hindered democracy alive amid a brutal Taliban insurgency.

The Islamist group vowed on Tuesday it would make the country "a graveyard" for the United States and would continue its "jihad" as long as American troops remained in the country.

"If America doesn't withdraw its troops from Afghanistan, soon Afghanistan will become another graveyard for this superpower in the 21st century," Zabiullah Mujahid, a spokesman for the Taliban in Afghanistan, said in a statement.

He added that America should think of an exit strategy "instead of continuing the war".

"As long as there is one US soldier in our land, and they continue to impose war on us, we, with a high morale will continue our jihad," Mujahid said.

Agencies contributed to this report.