London, Aug. 13: The UN has urged Afghanistan's neighbours to keep their borders open as the number of civilians fleeing the Taliban onslaught swells.

Many of those internally displaced have been arriving in Kabul, seeing the capital as their last safe refuge. Food shortages are "dire", the World Food Programme (WFP) said. It warned of a humanitarian catastrophe.

On Friday, the Taliban seized the country's second-largest city Kandahar, the latest provincial capital to fall. The southern city of 600,000 people was once the Taliban's stronghold and is strategically important as a trade hub.

The insurgents also took the nearby city of Lashkar Gah and now control about a third of Afghanistan's provincial capitals.

Many of those seeking safety in Kabul are sleeping on the streets. About 72,000 children are among those fleeing to the capital in recent days, according to Save the Children.

"We have no money to buy bread, or get some medicine for my child," a 35-year-old street vendor who fled northern Kunduz province after the Taliban set fire to his home told the BBC.

Makeshift camps have been established on scrubland on the outskirts of the capital, while many others have reportedly been sleeping in abandoned warehouses.

More than 1,000 civilians have been killed in Afghanistan in the past month, according to the UN.

The insurgent advance comes as US and other foreign troops withdraw after 20 years of military operations.

The US is to send nearly 3,000 troops to Kabul airport to evacuate a "significant" number of embassy staff on special flights. The UK is deploying 600 troops to support British nationals leaving the country. Staff at the British embassy have been reduced to a core team.

'From here where else do we run?'

By BBC's Yogita Limaye in Kabul

People are in disbelief about what's happened in a single day. Five provincial capitals - among them major cities - fell to the Taliban on Thursday.

In Kabul, thousands of people have been arriving - but this is a number that changes by the hours. They've left with very few belongings. These are people, who had homes and jobs, and shops and farms - and they just had to leave everything behind and try to run to safety.

Some of them have taken days, and these are dangerous journeys - past Taliban checkpoints and active frontlines - to get to Kabul. This is the last place many of them believe they can go to. They say, from here where else do we run?

They are angry at the government for being left to fend for themselves. The government says it is going to house them in mosques and provide them with relief - but there is not enough for everyone who is coming in.

There's anger too that the US and UK are evacuating their own citizens and leaving Afghans to their fate.

Why is Kandahar so important?

Kandahar is the Taliban's birthplace, and so taking control of the city is a significant prize for the militants. They had occupied the city's outskirts for a number of weeks before launching their attack on the centre.

On Wednesday, the Taliban breached Kandahar's central prison, and on Thursday, images on social media reportedly showed insurgents in the city centre.

A resident told the AFP news agency that government forces appeared to have withdrawn en masse to a military facility outside Kandahar.

Kandahar is considered strategically important because of its international airport, its agricultural and industrial output and its position as one of the country's main trading hubs.