Britain has warned all UK nationals in Afghanistan to leave the country immediately due to the "worsening security situation" as fighting intensifies.

The Foreign, Commonwealth and Development Office on Friday updated its website to advise against all travel to Afghanistan.

"All British nationals in Afghanistan are advised to leave now by commercial means. If you are still in Afghanistan, you are advised to leave now by commercial means because of the worsening security situation," it said.

The foreign office warned Britons not to rely on it for emergency evacuation, saying the assistance it could provide was "extremely limited".

The warning comes after the Taliban launched a major offensive to coincide with the withdrawal of US-led foreign forces after nearly two decades of conflict.

"Terrorists are very likely to try to carry out attacks in Afghanistan. Specific methods of attack are evolving and increasing in sophistication," the foreign office said.

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The Taliban now control vast swathes of rural Afghanistan and are challenging government forces in several cities, including Herat, near the western border with Iran, and Lashkar Gah and Kandahar in the south.

On Friday, the Islamist militants captured their first provincial capital since stepping up their offensive in May.

Zaranj, the capital of the southwest province of Nimroz, fell "without a fight", deputy provincial governor Roh Gul Khairzad told AFP.

Afghan journalists
Britain will offer sanctuary to Afghan media workers who operated with UK outlets and now face deadly threats from the Taliban, Foreign Secretary Dominic Raab said on Friday.

Responding in The Times to an open letter by Britain's leading newspapers and broadcasters calling on the government to offer refuge to Afghan colleagues, Raab said the scheme will allow relocation in "exceptional cases".

"We recognise the bravery of Afghan journalists and those that have worked tirelessly to support them in the pursuit of media freedom and the defence of human rights," he said.

"Your letter highlighted the threat faced by Afghan staff who have worked for your media organisations in Afghanistan, in particular the risk of reprisals they face from the Taliban from their association with the UK."

See Also: Taliban capture first Afghan provincial capital in blow to government

Every major British newspaper, as well as broadcasters Sky News and ITN, signed a letter to the government Thursday, warning the Taliban offensive sparked by the withdrawal of US-led forces put Afghan media workers at heightened risk.

The Islamist militants now control vast swathes of rural Afghanistan and are challenging government forces in several large cities, including Herat, near the western border with Iran, and Kandahar in the south.

The British media highlighted increasing violence against journalists including the murder in November last year of Mohammad Ilyas Dayee, who worked with British journalists, as well as the July killing of Indian photographer Danish Siddiqui from Reuters news agency.

The extension of the scheme to relocate Afghan workers comes amid pressure on the government to speed up its resettlement of translators and other staff who worked with British military during the two-decade conflict.

The government announced on Wednesday it aimed to relocate hundreds more Afghan translators and their families after senior military figures said it was not doing enough.