The UK government must show leadership over the unfolding crisis in Afghanistan, the Labour leader has said, as a former head of the British army called on the prime minister to launch an urgent humanitarian aid operation.

Keir Starmer said there was a risk of a global terror group taking control of the country after the withdrawal of British and US forces, while Richard Dannatt said there was still time to show Afghans they were not being completely abandoned.

“What I want to see is our government stepping up and leading this, and calling for an urgent meeting of Nato and an urgent UN security council meeting,” said Starmer.

“We have obligations to Afghanistan, we made promises to Afghanistan and we cannot walk away and let this turn into a humanitarian crisis, probably a refugee crisis as well. There is a real risk that international terrorism will take hold again in Afghanistan, so we can’t walk away and undermine the legacy of the last 20 years.”

He urged the government to respond positively to the latest call by the Afghan president, Ashraf Ghani, for international assistance to end the conflict in his country.

And he said the timing of the decision to pull out international forces from the country was “not right”, adding: “There appears to have been a miscalculation of the strength of the Taliban on the one side and the resilience of the Afghan troops and government on the other.”

Thousands of refugees fleeing the Taliban have been pouring into the capital Kabul as the militants continued their lightning advance across the country.

The collapse of Afghan government forces followed the decision of the Biden administration to withdraw all remaining US forces, prompting other allies – including the UK – to follow suit.

While Lord Dannatt said he shared the anger of many military veterans who served in Afghanistan at the way the country was being left to its fate, he insisted it was still not too late for the UK government to act.

“It is OK to extract our British citizens. What about mounting a humanitarian operation in Kabul to look after some of the refugees, to build some camps, bring in some humanitarian supplies?” he told BBC Breakfast.

“At least let’s show to the Afghan government we are not completely abandoning them and that we still stand side by side with them. It is quite possible to do that.

“I think our government should be thinking about that kind of response even though it has now pulled the plug on our wider military response.

“There may come a moment when our last troops have got to go but until that point, let us do what we can to help with the humanitarian crisis, even if we have given up helping on the military side.”

Boris Johnson said on Friday the current situation was the “inevitable logical consequence” of the decision by the Biden administration to withdraw all remaining US forces by the 20th anniversary next month of the 11 September 2001 terror attacks.

The prime minister insisted the sacrifices of the British armed forces who fought in the country had not been in vain, but said there was no question now of a “military solution” to halt the Taliban onslaught.

He said the government would use whatever political or diplomatic levers it could – including the UK overseas aid budget – to try to ensure that Afghanistan did not become a breeding ground again for international terrorism.

However, there was anger among MPs across the political spectrum at the way events had unfolded, with calls for parliament to be recalled from its summer break so they can discuss the crisis.