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.

Passengers walk in front of Hamid Karzai International Airport in Kabul, Afghanistan March 29, 2016.
Passengers walk in front of Hamid Karzai International Airport in Kabul, Afghanistan March 29, 2016. (Reuters Archive)

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

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.

READ MORE: What are the possible outcomes of the Afghanistan - Taliban conflict?

Taliban's actions won't help them gain international legitimacy -White House

Recent actions by the Taliban will not help them gain international legitimacy, the White House said on Friday, after insurgents from the group killed the government's senior media officer in Kabul.

"Our view is that, if the Taliban claim to want international legitimacy, these actions are not going to get them the legitimacy they seek," White House press secretary Jen Psaki said at a press briefing.

"They do not have to stay on this trajectory. They can choose to devote the same energy to the peace process as they are to their military campaign."

UN warns Afghanistan at 'dangerous turning point' as conflict spirals

Afghanistan is at a "dangerous turning point" as the Taliban continue to press their sweeping nationwide offensive against government forces, the UN special envoy for the war-torn country warned on Friday.

"Ahead lies either a genuine peace negotiation or a tragically intertwined set of crises: An increasingly brutal conflict combined with an acute humanitarian situation, and multiplying human rights abuses," Deborah Lyons told the Security Council.

"Today we have an opportunity, an opportunity to demonstrate the commitment of the UN Security Council and the international community that you represent to prevent Afghanistan from descending into a situation of catastrophe so serious that it would have few, if any, parallels this century," she said.

"Such a catastrophe would have consequences far beyond the borders of Afghanistan," added Lyons.

UN estimates indicate that over 1,000 people have been killed over the past month as the Taliban seeks to capture major government-held cities, and has claimed responsibility for a series of attacks on the capital, including one that targeted Afghanistan's defence minister.

READ MORE: NATO urges 'negotiated settlement' in Afghanistan amid Taliban challenge

Source: TRTWorld and agencies