United States President Donald Trump has opened the door to an increase in troops in Afghanistan as part of a retooled strategy for the region, overcoming his own doubts about America's longest war and vowing "a fight to win".

Key points: Donald Trump says safe havens for terrorists in Pakistan to become a focus

Donald Trump says safe havens for terrorists in Pakistan to become a focus President says he won't reveal details about when US will launch attacks

President says he won't reveal details about when US will launch attacks Defence Secretary has plans to send about 4,000 more troops to Afghanistan

In a prime-time televised address to the nation at Fort Myer in Virginia, Mr Trump said his new approach was aimed at preventing Afghanistan from becoming a safe haven for Islamist militants bent on attacking the US.

The President refused to set a deadline for pulling out of Afghanistan, saying a detailed timeline gives too much information to America's enemies.

"I will not say when we are going to attack, but attack we will," Mr Trump said.

"We will not talk about numbers of troops or our plans for further military activities.

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

The President said Pakistan and the safe havens there for terrorists would become a focus of US efforts to end the war in Afghanistan.

"We can no longer be silent about Pakistan's safe havens for terrorist groups," Mr Trump said.

A map of Afghanistan that details the areas the Taliban and Islamic State either have control or support. ( ABC News )

Mattis plans for 4,000 extra troops

Mr Trump left open the door to an increase in US troop numbers in Afghanistan as part of a new strategy for the region.

He did not say how many troops would be sent, but Defence Secretary James Mattis has plans on the table to send about 4,000 more to add to the 8,400 already deployed there.

A graph shows the rise and fall in numbers of US troops stationed in Afghanistan. ( ABC News )

"I have directed the chairman of the Joint Chiefs of Staff to make preparations to carry out the President's strategy," General Mattis said in a statement.

"I will be in consultation with the secretary-general of NATO and our allies — several of which have also committed to increasing their troop numbers."

Australian Defence Minister Marise Payne noted after the speech that in May, the ADF agreed to increase its train, advise and assist mission in Afghanistan, bringing the total number of personnel to about 300.

"We will examine the President's statement, consider any expectations of counterpart nations and engage in discussion with the US on those matters," she said.

Mr Trump said the US must seek an honourable and enduring outcome in Afghanistan worthy of the sacrifices that had been made.

"Our troops will fight to win," he said.

The speech, devoid of a lot of specifics, came after a months-long review of US policy in which Mr Trump frequently debated the future of US involvement in Afghanistan, where Taliban insurgents have been making territorial gains.

Afghan Ambassador to the US, Hamdullah Mohib, told AP the speech and the strategy outlined in it was "10 out of 10" and that Afghans heard "exactly what we needed to".

Mr Mohib said critiques of the speech for failing to disclose troop numbers were misguided and the focus on numbers detracts from the "real focus" on conditions and support needed for Afghanistan to succeed and achieve peace.

Mr Trump has long been sceptical of how the US is fighting the war in Afghanistan, which was launched by president George W Bush in October 2001 after the September 11 attacks on New York and Washington.

US troop numbers in Afghanistan November 2001: 1,300 US troops arrive in Afghanistan

November 2001: 1,300 US troops arrive in Afghanistan December 2001: The US force grows to 2,500 as troops hunt for Osama bin Laden

December 2001: The US force grows to 2,500 as troops hunt for Osama bin Laden March 2002: 7,200 US troops are in Afghanistan as the US lead the largest ground assault of the war to date

March 2002: 7,200 US troops are in Afghanistan as the US lead the largest ground assault of the war to date December 2003: Year ends with about 13,100 US troops in Afghanistan

December 2003: Year ends with about 13,100 US troops in Afghanistan February 17, 2009: There are about 38,000 US troops serving in Afghanistan

February 17, 2009: There are about 38,000 US troops serving in Afghanistan December 1, 2009: Obama announces troop increase, taking the US total to almost 100,000

December 1, 2009: Obama announces troop increase, taking the US total to almost 100,000 June 22, 2011: Barack Obama announces all of the additional US forces deployed in December 2009 will return home within 15 months

June 22, 2011: Barack Obama announces all of the additional US forces deployed in December 2009 will return home within 15 months Today: There are about 8,400 US troops in Afghanistan

Sorry, this video has expired US opens door to increased troops in Afghanistan, Trump says (Photo: Reuters)

Reuters/AP