In a speech to troops in southern Kandahar, birthplace of the Taliban, President Ashraf Ghani said Trump's first formal address as commander-in-chief late Monday showed that America was "with us, without any time limit".
"You cannot win this war," Ghani told the Taliban, calling on them to join talks and saying that Afghanistan wants peace with neighbouring Pakistan, long accused of fuelling the insurgency.
Trump pilloried US ally Pakistan for offering safe haven to "agents of chaos". He discarded his previous criticism of the 16-year-old war as a waste of time and money, admitting things looked different from "behind the desk in the Oval Office".
"My instinct was to pull out," Trump said as he spoke of his frustration with a war that has killed thousands of US troops and cost US taxpayers hundreds of billions of dollars.
But following months of deliberation, Trump said he had concluded "the consequences of a rapid exit are both predictable and unacceptable" -- a vacuum which terrorists "would instantly fill".
While Trump refused to offer detailed troop numbers, senior White House officials said he had already authorised his defence secretary to deploy up to 3,900 more troops to Afghanistan.
Trump warned that the approach would now be more pragmatic than idealistic. Security assistance to Afghanistan was "not a blank check" he said, adding: "We are not nation-building again. We are killing terrorists."
The US has grown increasingly weary of the conflict that began in October 2001 as a hunt for the 9/11 attackers and has turned into a vexed effort to keep Afghanistan's divided and corruption-plagued democracy alive amid a brutal Taliban insurgency.
In response to Trump, Taliban vowed to make the country "a graveyard" for the US and said it would continue its "jihad" as long as American troops remained in the country.
"If America doesn't withdraw its troops from Afghanistan, soon Afghanistan will become another graveyard for this superpower in the 21st century," spokesman Zabiullah Mujahid said in a statement.
Trump also indicated his single-minded approach would extend to US relations with Pakistan, criticised by successive US administrations for links with the Taliban and for harbouring leading jihadists -- like Osama bin Laden.
"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, warning that vital aid could be cut. "That will have to change and that will change immediately."
"We want peace from Pakistan, they cannot provide safe havens to anyone anymore," Ghani said in Kandahar.
But ahead of the speech Pakistan's military insisted the country has done all it can to tackle militancy.
Trump for the first time also left the door open to an eventual political deal with the Taliban.
"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".
His Secretary of State Rex Tillerson went further, saying the United States would "stand ready to support peace talks between the Afghan government and the Taliban without preconditions".
The Trump administration had originally promised a new Afghan plan by mid-July, but Trump was said to be dissatisfied by initial proposals to deploy a few thousand more troops.
His new policy will raise questions about what, if anything, can be achieved by making further deployments, or repeating the demands of previous administrations in more forceful terms.
In 2010 the United States had upwards of 100,000 US military personnel deployed to Afghanistan. Today that figure is around 8,400 US troops and the situation is as deadly as ever.
More than 2,500 Afghan police and troops have been killed already this year.
Trump's announcement comes amid a month of turmoil for his administration, which has seen several top White House officials fired and revelations that members of Trump's campaign are being investigated by a federal grand jury.
He sought in his address to convince Americans weary of his controversial off-the-cuff remarks of his seriousness.
"I studied Afghanistan in great detail and from every conceivable angle," he said -- though his effort was undermined when he later referred to Afghanistan's non-existent "Prime Minister".
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
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