Britain's Defence Secretary Ben Wallace said Tuesday it is "unlikely" evacuations from Afghanistan will be extended beyond August 31, as G7 leaders prepare to meet virtually to discuss the crisis.

The UK will chair the emergency talks among the group of wealthy countries later Tuesday, and has said it will urge the United States to extend the end-of-the-month deadline to complete the Afghan airlifts.

France has also called on Washington to push back the timeline.

But hours before the summit, Wallace acknowledged that both US President Joe Biden and Taliban leaders now in power in Kabul had cast serious doubt on evacuations continuing into September.

"I think it is unlikely," he told Sky News. "Not only because of what the Taliban has said, but if you look at the public statements of President Biden I think it is unlikely.

"It is definitely worth us all trying, and we will."

Biden has set an August 31 deadline to finish the chaotic airlift organised by thousands of temporarily deployed US and UK troops, but has left the door open to an extension if needed.

However, a spokesman for the Taliban warned Monday the hardline Islamist group would not agree to any extension, calling the issue a "red line", with any delay viewed as "extending occupation".

"If the US or UK were to seek additional time to continue evacuations -- the answer is no. Or there would be consequences," Taliban spokesman Suhail Shaheen told Sky News.

'Next phase'

Britain has continued to evacuate Western citizens and some Afghans from the capital, with Wallace warning the security situation was getting "more and more dangerous" as August 31 approaches.

The defence ministry said 8,458 people have been evacuated by the UK since August 13, with nine military flights leaving Kabul in the last 24 hours.

More than half -- 5,171 -- are Afghans eligible to relocate to Britain under its programme to protect those who aided its military and civilian officials during their two-decade involvement in Afghanistan.

An individual on the UK's no-fly anti-terrorism watchlist arrived as part of the evacuation, the interior ministry confirmed.

A spokesman said the individual was identified "as part of the rigorous checks process" and that after further investigation was deemed "not a person of interest to the security agencies or law enforcement".

Meanwhile, on the eve of the G7 meeting, British Prime Minister Boris Johnson said the leaders of the world's top economies should also have an eye on the "next phase" for Afghanistan.

His office said he will urge fellow leaders to step up support for refugees and to restate their "commitment to safeguarding the gains made in Afghanistan over the last 20 years -- in particular on girls' education and the rights of women and minorities".

Britain currently chairs the G7, which also comprises Canada, France, Germany, Italy, Japan and the United States.