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Reversing his past calls for a speedy exit, President Donald Trump recommitted the United States to the 16-year-old war in Afghanistan Monday night, declaring U.S. troops must "fight to win." He pointedly declined to disclose how many more troops will be dispatched to wage America's longest war.

In a prime-time address to unveil his new Afghanistan strategy, Trump said the U.S. would shift away from a "time-based" approach, instead linking its assistance to results and to co-operation from the beleaguered Afghan government, Pakistan and others. He insisted it would be a "regional" strategy that addressed the roles played by other South Asian nations — especially Pakistan's harbouring of elements of the Taliban.

"America will work with the Afghan government as long as we see determination and progress," Trump said. "However, our commitment is not unlimited, and our support is not a blank check."

Although Trump insisted he would "not talk about numbers of troops" or telegraph military moves in advance, he hinted that he'd embraced the Pentagon's proposal to boost troop numbers by nearly 4,000, augmenting the roughly 8,400 Americans there now.

"We will ask our NATO allies and global partners to support our new strategy, with additional troop and funding increases in line with our own. We are confident they will," Trump said in comments echoed by Defence Secretary Jim Mattis.