US President Donald Trump said 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 on Monday evening - his first as commander-in-chief, Trump said his "original instinct was to pull out", alluding to his long-expressed view before becoming president that Afghanistan was an unsolvable quagmire requiring US withdrawal.

But since taking office, Trump said, he had determined that withdrawing could create a vacuum that groups such as al-Qaeda and the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) could "instantly fill".

TIMELINE: US intervention in Afghanistan from 2001 to 2017

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

Trump warned that the approach would now be more pragmatic than idealistic. Security assistance to Afghanistan was "not a blank cheque" 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," he said.

OPINION: ISIL in Afghanistan - A growing threat

Though his speech was billed as an announcement of his updated Afghanistan policy, Trump offered few specific details.

He did not provide the number of additional troops that would be sent to the war, though US officials said before the speech they expect him to go along with a Pentagon recommendation for nearly 4,000 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."

Al Jazeera's Patty Culhane, reporting from Washington, DC, said it was unusual that Trump did not disclose the number of extra troops.

"There are going to be questions and calls from members of congress and members of the public to say, 'the American public has a right to know just how many are fighting in their name and how much it's going to cost'," she said.

There are roughly 8,400 American forces in Afghanistan. At its peak, the US had roughly 100,000 forces there under the Obama administration in 2010-2011.

READ MORE: World reacts to Trump's new strategy on Afghan war

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

"I share the American people's frustration," Trump said at the Army's Joint Base Myer-Henderson Hall, in Arlington, Virginia, across the Potomac River from the White House.

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

Tougher approach to Pakistan

Trump also laid out a tougher approach to US policy towards Pakistan which he accused of giving "safe haven to agents of chaos, violence and terror".

"We can no longer be silent about Pakistan's safe havens for terrorist organisations," Trump said, outlining the strategy in South Asia.

"Pakistan has much to gain from partnering with our effort in Afghanistan. It has much to lose by continuing to harbour criminals and terrorists."

OPINION: What should the US' next move be in Afghanistan?

Trump suggested that military and other aid to Washington's nuclear-armed ally is at stake.

"We have been paying Pakistan billions and billions of dollars at the same time they are housing the very terrorists that we are fighting," he said. "That will have to change, and that will change immediately."

Trump said the US wanted India to help more with Afghanistan, especially in the areas of economic assistance and development.

Open to Taliban deal

Trump also left the door open to an eventual political deal with the Taliban in his address.

"Someday, after an effective military effort, perhaps it will be possible to have a political settlement that includes elements of the Taliban in Afghanistan," he said.

"But nobody knows if or when that will ever happen," he added, before vowing that "America will continue its support for the Afghan government and military as they confront the Taliban in the field".

It was for the people of Afghanistan "to take ownership of their future," the president said.

"We are a partner and a friend, but we will not dictate to the Afghan people how to live or how to govern their own complex society. We are not nation building again. We are killing terrorists."

Al Jazeera's Jennifer Glasse, reporting from Kabul, said that Trump's message focused on military support.

"I think the real question here for Afghans is going to be: 'Is this really a new strategy, or more of the same?'" she said.

As the Taliban has ramped up attacks in recent months, Afghans would have been seeking specific details, Glasse said, but they were missing.

"For the United States, it may look like a new strategy. Whether it's going to look like that to the Afghans is the real question," said Glasse.