WASHINGTON, Aug. 21 (Xinhua) -- U.S. President Donald Trump said on Monday that a rapid exit of the U.S. 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.

Trump made the announcement in a prime-time address at 9 p.m. local time (0100 GMT, Tuesday) from Fort Myer in Virginia. He did not talk about the widely anticipated increase of U.S. troops in Afghanistan.

In the over-30-minute speech, Trump ruled out a quick withdrawal of the U.S. troops, saying that it would have unacceptable consequences and "create a vacuum" that the Islamic State 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 U.S. troops and another 5,000 troops of the North Atlantic Treaty Organization (NATO) in Afghanistan to train and assist Afghan forces against the Taliban, and conduct counter-terrorism missions.

Ahead of Trump's speech, U.S. media expected the president to authorize 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.