Britain’s defence secretary has warned that Afghanistan risks descending into civil war and becoming a failed state as the Taliban captures large cities and gains ground in the wake of the hasty US troop withdrawal.

The Islamist militia is now in control of Kandahar, Afghanistan’s second-biggest city, and Lashkar Gah in Helmand province after weeks of fierce fighting.

The insurgents now control at least 14 of Afghanistan’s 34 provincial capitals, and are encircling Kabul in an effort to topple President Ashraf Ghani’s weakened government.

Ben Wallace said he believed Afghanistan could once again become a haven for international terrorists such as al-Qaeda, whose presence in the country prompted the US-led invasion to oust the Taliban from power almost 20 years ago.

“I’m absolutely worried that failed states are breeding grounds for those types of people,” the UK defence secretary told Sky News on Friday.

Wallace criticised Washington’s decision to stand by its target of withdrawing the remaining troops by the end of the month, saying: “I felt this was not the right time or decision to make because of course al-Qaeda will probably come back”.

He also told the BBC that Afghanistan was “heading towards civil war”.

Tom Tugendhat, chair of the UK foreign affairs committee, also lamented the troop exodus. “We’ve just pulled the rug from under these guys,” he told the BBC. “We’ve taken away their air support, we’ve taken away their logistics and said, ‘Go on then, let’s see how you do’.”

The US and UK said they would send troops to Kabul to evacuate embassy staff as fears mounted that the capital could come under imminent attack if a political settlement was not reached. The Taliban has shown little appetite for such a deal.

The Pentagon is sending 3,000 troops in the next two days and a back-up detachment of 3,500 soldiers to Kuwait in case the security situation deteriorates further. The UK will deploy a further 600 soldiers to speed up evacuations of some diplomatic personnel and Afghans who worked with British forces.

After storming across much of Afghanistan’s countryside in recent months, the Taliban have in the space of a week toppled provincial capital after provincial capital, dramatically altering the balance of power in the country.

After recently capturing Kandahar, Herat and Lashkar Gah, only Mazar-i-Sharif, a northern stronghold of local anti-Taliban warlords, and Jalalabad, to the south of Kabul, are the last big towns resisting the Islamists outside the capital. The Taliban now controls well over half of the country’s territory.

Even as the Islamist militants pressed their offensive, Taliban representatives were in Qatar for talks with a range of governments including the US, UK, Pakistan, China, India and several others.

In a statement issued late on Thursday, Doha said the participants in the talks had agreed on the need to “accelerate efforts to reach a political settlement and comprehensive ceasefire as quickly as possible”.

Zalmay Khalilzad, the US special representative for Afghanistan, demanded “an immediate end to attack against cities” and warned that “a government-imposed by force will be a pariah state”.

However, analysts doubted that the Taliban leaders in Qatar had control over the insurgent group’s ground troops, many of whom were operating autonomously in their local regions.

“The question is to what extent are Mullah Baradar [the Taliban co-founder] and his crew in Doha are going to be able to shape the mind of hardened fighters who have never been in an aeroplane,” said Rudra Chaudhuri, a lecturer at the King’s College London’s department of war studies.

“It doesn’t seem to me that a motorcycle riding Taliban leader with a Kalishnikov is very interested in what Baradar says.”

Additional reporting by Sarah Provan in London