The UK is sending military personnel to help evacuate Britons from Afghanistan as the security situation in the country rapidly deteriorates.

The move was authorised by the defence secretary on Thursday morning.

Ben Wallace said 600 troops will be sent to Kabul on a "short-term basis" in response to the increasing violence across the country. They are expected to arrive in the coming days.

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Will Kabul fall to the Taliban?

The decisions came as Taliban captured Kandahar and Herat - Afghanistan's second- and third-largest cities respectively, and US intelligence warned Kabul could fall within 90 days.

The capture of Kandahar marks the biggest prize yet for the Taliban, which has now taken 12 of Afghanistan's 34 provincial capitals.

Officials said Kandahar fell on Thursday night and that government officials and their entourage managed to flee to the airport to escape the city by air.

The earlier seizure of Herat marked one of the biggest prizes yet for the Taliban.

More on Afghanistan

Fighters rushed past the Great Mosque in the historic city - which dates to 500 BC and was once a spoil of Alexander the Great - and seized government buildings.

Twenty years on, Sky's Stuart Ramsay returns to the scene of a crisis happening all over again

Twenty years on, Sky's Stuart Ramsay returns to the scene of a crisis happening all over again

Mr Wallace said: "I have authorised the deployment of additional military personnel to support the diplomatic presence in Kabul, assist British nationals to leave the country and support the relocation of former Afghan staff who risked their lives serving alongside us," said Mr Wallace.

"The security of British nationals, British military personnel and former Afghan staff is our first priority. We must do everything we can to ensure their safety."

The Ministry of Defence has characterised the move as part of the ongoing withdrawal of NATO forces, but in reality it is an unplanned emergency response to the rapidly deteriorating security situation in Afghanistan as the Taliban rampage across the country.

The British soldiers will also assist with the acceleration of the Afghan Relocation and Assistance Policy.

"This will help to make sure interpreters and other Afghan staff who risked their lives working alongside UK forces in Afghanistan can relocate to the UK as soon as possible," the Ministry of Defence said in a statement.

The number of staff working at the British Embassy in Kabul has also been significantly reduced to a "core group", the government has announced - the remaining staff will focus on consular help to anyone wanting to leave the country.

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The British ambassador to Afghanistan, Sir Laurie Bristow, will remain in Kabul but will relocate to a more secure location.

The US has also announced that it is deploying some 3,000 additional troops to help the departure of its embassy staff.

The American embassy will remain open, although personnel will be reduced to a "core diplomatic presence".

Analysis by Martha Kelner, US correspondent

This is fast becoming a chaotic end to America’s 20 year presence in Afghanistan as 3,000 troops are rushed in to facilitate the extraction of US embassy personnel from the capital, Kabul.

Understandably, many people here are now recalling the 1975 fall of Saigon when US staff were airlifted to safety from the roof of the embassy, a situation the US was desperate to avoid.

When President Biden announced the withdrawal of US troops in April, the biggest fear was that provincial cities in Afghanistan would fall one after the other and that is exactly what is happening as the Taliban advance at a speed even the most gloomy predictions did not forecast.

Intelligence officials here suggested privately in June that Kabul could be taken within six months, a timeline that was later revised to 90 days but the situation in Afghanistan and the lack of resistance being put up in many areas to repel the Taliban means it could be taken even sooner than that. The momentum now is solely with the Taliban and it is almost certainly already too late to reverse their gains.

With 2,312 US troops lost in combat in this Afghanistan war and more than $1trillion spent on the conflict and all the positive advancements now crumbling, the question is, what exactly was the last 20 years all for?

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Afghanistan withdrawal a 'catastrophic mistake'

On Wednesday, David Miliband, president of the International Rescue Committee, warned the West not to take its eyes off Afghanistan as the Taliban's forces continue to make gains after British and US troops were withdrawn.

Conservative MP and chairman of the Foreign Affairs Committee Tom Tugendhat suggested the UK's withdrawal from Afghanistan is a mistake.

"We can turn this around. We need to. Investing in ourselves, our allies and partners has never been easier or more important. This is a choice. So far we're choosing to lose," he posted on social media.