The UK defence secretary has criticised the US decision to leave Afghanistan as a “mistake” that has handed the Taliban “momentum”.

Speaking to Sky News, Ben Wallace warned that “the international community will probably pay the consequences” and said he was worried al-Qaida would regain a base in Afghanistan.

He confirmed UK plans to deploy 600 troops to Afghanistan to help 3,000 people including interpreters and British passport holders to leave, as officials said on Friday that the Taliban had captured Afghanistan’s second biggest city, Kandahar, as well as Lashkar Gah, the capital of Helmand province in the south.

Wallace said the withdrawal agreement negotiated in Doha, Qatar, by the Trump administration was a “rotten deal” which the UK tried to resist.

He said the UK had no choice but to pull troops out, because the international community had to act together. “When the United States as the framework nation took that decision, the way we were all configured meant that we had to leave,” Wallace said.

Asked how big a mistake it was to withdraw troops, Wallace said: “At the time of the Trump deal with, obviously the Taliban, I felt that was a mistake to have done it that way. We will all, in the international community probably pay the consequences of that.”

He added: “I’ve been pretty blunt about it publicly and that’s quite a rare thing when it comes to United States decisions, but strategically it causes a lot of problems and as an international community, it’s very difficult for what we’re seeing today.”

Asked about the threat of Afghanistan becoming a base for terrorism, Wallace said: “I’m absolutely worried that failed states are breeding grounds for those types of people. It’s why I felt this was not the right time or decision to make because al-Qaida will probably come back.”

Wallace added: “I think the deal that was done in Doha was a rotten deal. It effectively told a Taliban that wasn’t winning that they were winning, and it undermined the government of Afghanistan and now we’re in this position where the Taliban have clearly the momentum across the country.

“The United States are leaving, we are leaving alongside them, and that leaves a very, very big problem on the ground developing with the Taliban, obviously with the momentum and it’s not what we probably would have liked. I did try after the announcement, to see if we can bring together the international community. And I’m afraid most in that community weren’t particularly interested.”