British forces have a matter of days to complete their humanitarian airlift in Afghanistan after Boris Johnson failed to persuade Joe Biden and other G7 leaders to extend a deadline for US forces to leave Kabul beyond 31 August.

Speaking after the virtual G7 gathering, Johnson said it had agreed a “roadmap for future engagement with the Taliban”, on the assumption they would be governing Afghanistan.

The “number one condition” for engagement with the Taliban would be guaranteed safe passage for people who wanted to leave the country both up to the end of August and beyond.

This could prove contentious for the Taliban, with its spokesperson Zabihullah Mujahid using a press conference on Tuesday to say Afghans should stop going to Kabul airport immediately.

Johnson said that other conditions needed for international funds to be accessible would be for Afghanistan to not “lurch back into being a breeding ground for terror” or resume its drugs trade, while the Taliban would also have to allow girls to be educated up to the age of 18.

“What we’ve done today at the G7 is we’ve got together the leading western powers and agreed not just a joint approach to dealing with the evacuation, but also a roadmap for the way in which we’re going to engage with the Taliban, as there probably will be a Taliban government in Kabul,” the prime minister said.

His comments were reflected in a joint G7 communique, which called the evacuation programme the “immediate priority”.

It added: “We express our grave concern about the situation in Afghanistan and call for calm and restraint to ensure the safety and security of vulnerable Afghan and international citizens, and the prevention of a humanitarian crisis. We call for adherence to obligations under international human rights law, including the rights of women, girls and minority groups, and that international humanitarian law is upheld in all circumstances.”

Johnson had been expected to ask Biden, the US president, about the possibility of allowing more time to remove people.

However, in a Pentagon briefing, Adm John Kirby, the Pentagon press secretary, said: “There’s been no change to the timeline of the mission, which is to have it done by the end of the month.”

In their own press conference on Tuesday, the Taliban confirmed 31 August as the end date. “All people should be removed prior to that date,” said Mujahid. “After that we do not allow them. It will not be allowed in our country. We will take a different stance.”

Asked if he had failed to secure an extension beyond 31 August, Johnson dodged the direct question to instead praise what he called an “extraordinary airlift”.

However, he obliquely accepted that an extension had not been possible: “You’ve heard what the president of the United States has had to say. You’ve heard what the Taliban have said. I think you’ve got to understand the context in which we’re doing this. We’re confident we can get thousands more out, but the situation at the airport is not getting any better.”

This will in effect give only a few days for UK forces to complete their work. UK officials say that British troops will need to leave before the final US contingents leave on 31 August, with the evacuations stopping before then.

Britain has evacuated 8,600 people from Afghanistan in the past 10 days, including more than 2,000 in the previous 24 hours, according to Ben Wallace, the defence secretary.

The Pentagon has said 21,600 people in total flew out of Kabul airport in the past 24 hours, 12,700 on US military transport planes, with the rest taken by other nations.

As the G7 meeting began, Bond, an organisation representing more than 400 British aid groups, including more than 30 working directly in Afghanistan, warned against policies that could hamper efforts to assist Afghans, such as threats of sanctions against the Taliban.

The British & Irish Agencies Afghanistan Group, an umbrella organisation for NGOs working in the country, said much more needed to be done to remove people beyond the end of August.

“While we welcome the efforts being made to evacuate Afghan civil society activists from Kabul airport, huge numbers will be left behind once the airlift ends,” said Elizabeth Winter, its executive director.

“The UK’s offer to resettle just 5,000 Afghans this year is inadequate and lacks the urgency needed to help the many thousands of people at risk.”