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.

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 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.