“As far as I’m concerned, they are dead,” he told White House reporters on Monday.
Over the weekend Mr Trump cancelled secret plans to host a Taliban delegation in the US after the militant group admitted killing a US soldier.
The two sides had appeared close to a deal and the Taliban said the US would “lose the most” for cancelling talks.
The US president has made withdrawing US troops from Afghanistan a key foreign policy aim, but asked about the 14,000 US troops still there he said: “We’d like to get out but we’ll get out at the right time.”
Mr Trump had been due to host the Taliban as well as Afghan president Ashraf Ghani at the Camp David presidential retreat before abruptly cancelling.
“They thought that they had to kill people to put themselves in a little better negotiating position”, he told reporters, calling the attack “a big mistake”.
Mr Ghani’s office has praised the “sincere efforts” made by negotiators.
What did Mr Trump say?
“We had a meeting scheduled. It was my idea and it was my idea to terminate it. I didn’t even discuss it with anyone else,” Mr Trump said as he departed the White House for a political rally in North Carolina.
“I cancelled Camp David on the basis that they did…