British nationals in Afghanistan are being told to leave immediately as the country moves into what the UN has described as a “deadlier and more destructive phase”.

The Foreign, Commonwealth and Development Office on Friday evening issued an advisory against all travel to Afghanistan.

It added: “If you are still in Afghanistan, you are advised to leave now by commercial means because of the worsening security situation.”

It comes after Taliban fighters recaptured Zaranj in southern Nimroz, the first provincial capital to have fallen to the extremist group since it briefly held Kunduz in the north in 2016.

The Taliban posted images on social media that showed insurgents inside the local airport, as well as posing for photographs at the entrance of the city.

Nimroz is a sparsely populated region that is mostly desert, and the provincial capital has about 50,000 residents.

At least 1,000 civilians have been killed in Afghanistan during the past month, and more than half of Afghanistan’s 421 districts and district centres are now in Taliban hands, along with lucrative border crossings into Iran, Tajikistan and Pakistan.

At a special meeting of the UN Security Council on Friday, Deborah Lyons, the UN envoy to Afghanistan, said the fighting and resultant human toll were worsening.

“The war in Afghanistan has entered a new, deadlier, and more destructive phase,” she said.

“The provincial capitals of Kandahar, Herat, and Lashkar Gah in particular have come under significant pressure.

“This is a clear attempt by the Taliban to seize urban centres with the force of arms.”

She added: “The human toll of this strategy is extremely distressing – and the political message is even more deeply disturbing.”

She said that 104 civilians were killed in just 10 days in Lashkar Gah, the capital city of Helmand province, as insurgents sweep across the country following the withdrawal of foreign troops earlier this year.

Also on Friday, Afghan government forces joined US aircraft in attacking Taliban positions in Helmand, where the militants control nine out of the 10 city districts.

Meanwhile, the Taliban assassinated Dawa Khan Menapal, the chief of the Afghan government’s press operations for local and foreign media and previously a deputy spokesman for President Ashraf Ghani.

Mr Menapal was murdered while in his car during Friday prayers in Kabul, the Afghan capital.

It comes days after an attempt to kill the country’s acting defence minister, Bismillah Khan Mohammadi, in a Taliban bombing that left eight people dead and 20 wounded.

The minister was unharmed.

Last month the Taliban took control of the town of Spin Boldak, near one of the country’s busiest border crossings with Pakistan.

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Thousands of people cross daily, alongside a steady stream of trucks bringing goods to the land-locked country from the Arabian Sea port city of Karachi.

However, the Taliban closed the crossing on Friday over a visa dispute, claiming Pakistan was abiding by Kabul government requirements for Afghans travelling into Pakistan. Previously, travel documents were rarely required.

“The border will stay closed until Pakistan allows all Afghans to cross on the bases of our old procedure,” said a Taliban statement.

On Friday, at least 1,500 people were waiting on both sides to pass through, with more than 600 trucks, many loaded with perishable fresh foods, backed up on both sides of the border.