US President Donald Trump said on Monday that a rapid exit of the US troops from Afghanistan was "unacceptable" and his new Afghanistan strategy will shift from a time-based approach to one based on conditions on the ground, China's Xinhua news agency reported.

Trump made the announcement in a prime-time address at 9pm local time (0100 GMT, Tuesday) from Fort Myer in Virginia.

He did not talk about the widely anticipated increase of US troops in Afghanistan.

In the over-30-minute speech, Trump ruled out a quick withdrawal of the US troops, saying that it would have unacceptable consequences and "create a vacuum" that the Daesh and al-Qaida would fill.

The president's national address came after a lengthy meeting with his national security team at Camp David on Friday.

He also made it clear that he would not "talk about numbers of troops or our plans for further military activities."

Currently, there are about 8,400 US troops and another 5,000 troops of the North Atlantic Treaty Organisation (NATO) in Afghanistan to train and assist Afghan forces against the Taliban, and conduct counter-terrorism missions.

Ahead of Trump's speech, US media expected the president to authorise the deployment of up to 4,000 additional troops to Afghanistan.

Instead of being specific on troop numbers, Trump outlined the pillars of his strategy for Afghanistan and the South Asian region, saying that the United States was focused on "killing terrorists," not nation building.

He said that the new strategy would be based on conditions on the ground, not timing.

While reassuring the Afghan government about the US commitment of cooperation, Trump also warned that the US support was "not a blank check."

"The government of Afghanistan must carry their share of the military, political and economic burden," said Trump.

The new Afghanistan strategy came at a time when senior US officials warned of a dire security situation in Afghanistan.

In a congressional hearing in June, US Defence Secretary Jim Mattis said the United States was still "not winning" the war in Afghanistan, the longest in US history.

Trump, a long-time critic of how the United States is fighting the war in Afghanistan, announced a review of the US strategy on Afghanistan soon after taking office in January.

Then US President George W. Bush ordered a dispatch of US troops to fight the war in Afghanistan in October 2001, after the 9/11 terrorist attacks.

Since then, about 2,400 US forces reportedly have died in Afghanistan.

-- BERNAMA