President Donald Trump says the United States must continue fighting in Afghanistan to avoid the “predictable and unacceptable” results of a rapid withdrawal from the country, where it has been at war for 16 years.

In a prime-time address to the nation, President Trump said his “original instinct was to pull out”, alluding to his long-expressed view before becoming president that Afghanistan was a unsolvable quagmire requiring a fast US withdrawal.

Since taking office, the President said, he’d determined that approach could create a vacuum that terrorists including al-Qaida and the Islamic State could “instantly fill”.

“I concluded that the security threats we face in Afghanistan and the broader region are immense,” President Trump said.

Though his speech was billed as an announcement of his updated Afghanistan policy, he offered few specifics about what it would entail.

He did not provide a number of additional troops that will be sent to the war, though US officials said ahead of the speech they expect him to go along with a Pentagon recommendation for nearly 4000 new troops.

“We will not talk about numbers of troops or our plans for further military activities,” President Trump said.

“Conditions on the ground, not arbitrary timetables, will guide our strategy from now on,” he said.

“I’m not saying when we’re going to attack, but attack we will.”

President Trump: “I am not saying when we are going to attack, but attack we will” https://t.co/meA2dWIOPP

— CNN (@CNN) August 22, 2017

There are roughly 8400 American forces in Afghanistan now.

At its peak, the US had roughly 100,000 forces there, under the Obama administration in 2010-2011.

President Trump said the American people are “weary of war without victory”.

“I share the America people’s frustration,” President Trump said at the Army’s Joint Base Myer-Henderson Hall, in Arlington, Virginia.

Still, he insisted that “in the end, we will win”.

President Trump took a hard line on Pakistan, promising to crack down on Afghanistan’s neighbour from harbouring of terrorist and militant groups.

The US has now been fighting in Afghanistan for 16 years. Photo: Getty

He said Pakistan has “much to gain” from partnering with the US, but also warned “it has much to lose by continuing to harbour criminals and terrorists.”

“They are housing the very terrorists we are fighting,” the President said, noting that the US gives Pakistan billions of dollars in aid.

“That will have to change and that will change immediately. No partnership can survive a country’s harbouring of terrorists,” he said.

Trump also vowed to pressure regional powerhouse India to increase its support for Afghan economic development.