WASHINGTON -- President Donald Trump said Thursday the U.S. plans to withdraw more than 5,000 American troops from Afghanistan and then will determine further drawdowns in the longest war in American history.

Trump's comment comes as a U.S. envoy is in his ninth round of talks with the Taliban to find a resolution to the nearly 18-year-old war. The president said the U.S. was "getting close" to making a deal, but the outcome is uncertain.

"Who knows if it's going to happen," Trump told Fox News Radio's "The Brian Kilmeade Show."

Trump did not offer a timeline for withdrawing troops. The Pentagon has been developing plans to withdraw as many as half of the 14,000 U.S. troops still there, but the Taliban want all U.S. and NATO forces withdrawn.

"We're going down to 8,600 (troops) and then we'll make a determination from there," Trump said, adding the U.S. is going to have a "high intelligence" presence in Afghanistan going forward.

Reducing the U.S. troop level to 8,600 would bring the total down to about where it was when Trump took office in January 2017. According to the NATO/Resolute Support mission, the U.S. had 9,000 troops in Afghanistan in 2016, during the Obama administration, and 8,000 in 2017.

Trump has called Afghanistan -- where the Taliban harbored members of the al-Qaida network responsible for 9/11 -- the "Harvard University of terror."

If terror groups ever attacked America from Afghanistan again, "we will come back with a force like they've never seen before," Trump said. But he added: "I don't see that happening."

Al-Qaida insurgents used Afghanistan as a base from which to plan the Sept. 11, 2001, attack on the United States. A month later, U.S. troops invaded Afghanistan, where they have remained ever since, making it the longest war in American history. More than 2,400 American service members have died in the conflict.

A Taliban spokesman also has said they're close to a final agreement. But even as the talks go on, there are persistent attacks by the Taliban across Afghanistan, and an affiliate of the Islamic State group, also known as ISIS, has taken root in the country and is expanding its base.

"Taliban and ISIS are still potential threats for the national security of Afghanistan and the US. The Afghan government strongly believes any reduction of US forces in Afghanistan will be based on conditions on the ground," Sediq Seddiqi, a spokesman for Afghan President Ashraf Ghani, said in a statement.

He added the Afghan government hopes Afghanistan and the U.S. will continue to fight international terrorism together as the countries have done for many years.

"The level of threats from the Taliban and ISIS has increased in Afghanistan and it is therefore needed now more than any other time to counter them together and make sure that we will not leave any gap that would give the Taliban an opportunity to turn this country again in to a safe haven for international terrorists," he said.

The top U.S. military officer said Wednesday it's too early to talk about a full American troop withdrawal from Afghanistan. Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, told Pentagon reporters any U.S. deal with the Taliban will be based on security conditions on the ground and Afghan forces aren't yet able to secure the country without help from allied forces.

"I'm not using the withdraw word right now," Dunford said. "It's our judgment that the Afghans need support to deal with the level of violence" in the country today.

Afghanistan's government expects U.S. envoy Zalmay Khalilzad will soon update officials in Kabul on the progress of peace talks with the Taliban.

Even if Khalilzad is able to close a deal, it will remain for the Afghan government to negotiate its own peace agreement with the Taliban. Part of those talks will be determining a role for the Taliban in governing a country it ruled before U.S. forces invaded in October 2001.

Sen. Lindsey Graham, R-S.C., a key Trump supporter, and retired Gen. Jack Keane, a former Army vice chief of staff, have urged caution about trusting the Taliban to keep Afghanistan from becoming a launch pad for international terror attacks again.

"The United States cannot contract out the American people's security to the Taliban who, in exchange for a U.S. withdrawal, simply 'promise' to guarantee that al-Qaida and ISIS (in Afghanistan) are denied haven," they wrote in an op-ed Wednesday in The Washington Post.

Graham and Keane said they fear a U.S. withdrawal will not end the war and could start a new civil war as Afghan forces feel betrayed and abandoned and the Afghan government is severely undermined and weakened.

"The United States should never outsource its national security to anyone, especially the Taliban," they cautioned. "We cannot rely on the Taliban for security; we have lost too many soldiers at Taliban hands for that. The Afghan war must end on our terms, not the Taliban's."

Moreover, they said U.S. national security interests require any deal the United States signs with the Taliban should allow for a "meaningful U.S. counterterrorism capability, coupled with a robust intelligence apparatus" to deal with threats from terror groups.