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

In a prime-time address to the nation, 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, Trump 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," Trump said.

Though his speech was billed as an announcement of his updated Afghanistan policy, Trump 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," Trump said.

"Conditions on the ground, not arbitrary timetables, will guide our strategy from now on."

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.

Trump said the American people are "weary of war without victory".

"I share the America people's frustration," Trump said at the Army's Joint Base Myer-Henderson Hall, in Arlington, Virginia.

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