WASHINGTON — President Donald Trump’s plan to end America’s longest war and eliminate Afghanistan’s rising extremist threat involves sending up to 3,900 additional U.S. troops, senior officials said Tuesday. The first deployments could take place within days.

In a national address Monday night, Trump reversed his past calls for a speedy exit and recommitted the United States to the 16-year-old conflict, saying U.S. troops must “fight to win.” He warned against repeating what he said were mistakes in Iraq, where an American military withdrawal led to a vacuum that the Islamic State group quickly filled.

Trump would not confirm how many more service members he plans to send to Afghanistan, which may be the public’s most pressing question about his strategy. In interviews with television networks Tuesday, Vice President Mike Pence similarly wouldn’t give any clear answer, but he cited Pentagon plans from June calling for 3,900 more troops.

“The troop levels are significant, and we’ll listen to our military commanders about that,” Pence said.

Although the Pentagon’s plans are based on 3,900 additional troops, the exact number will vary as conditions change, senior U.S. officials said. Those officials weren’t authorized to speak publicly on the figures and demanded anonymity.

They said the Pentagon has told Trump it needs the increase, on top of the roughly 8,400 Americans now in the country, to accomplish Trump’s objectives.

Those goals, he said Monday night, include “obliterating ISIS, crushing al-Qaida, preventing the Taliban from taking over Afghanistan and stopping mass terror attacks against America before they emerge.”

Speaking to reporters in Iraq, Defense Secretary Jim Mattis declined to confirm a precise number Tuesday, saying he was waiting for more input from Gen. Joseph Dunford, America’s top military official. Mattis said he will “reorganize” some U.S. troops in Afghanistan to reflect the new strategy.

Meanwhile, the top U.S. commander for the Middle East said he expects the first reinforcements to arrive “pretty quickly,” within days or weeks.

“What’s most important for us now is to get some capabilities in to have an impact on the current fighting season,” Gen. Joseph Votel, who spent last weekend in Afghanistan, told reporters traveling with him to Saudi Arabia on Tuesday.