US troops have begun arriving in Afghanistan to help evacuate thousands of people, including embassy staff, and Afghans and their families who worked for them as a sweeping Taliban offensive draws ever nearer to Kabul.

Diplomats and nationals from a host of western countries are scrambling to leave the capital, with insurgent fighters now camped just 50km (30 miles) away after a campaign that has seen provincial capitals swiftly fall.

US embassy staff were ordered to begin shredding and burning sensitive material, as units from a planned re-deployment of 3,000 American troops started arriving to secure the airport and oversee the evacuations.

Pentagon spokesman John Kirby said that most of the troops would be in place by Sunday and “will be able to move thousands per day” out of Afghanistan. “Capacity is not going to be a problem,” he said.

Helicopters have been flitting back and forth between Kabul’s airport and the sprawling US diplomatic compound in the heavily fortified Green Zone – 46 years after Americans were airlifted out of Saigon, signalling the end of the Vietnam war.

The UK said about 600 troops would be deployed on a short-term basis to support British nationals leaving. Earlier on Friday, many countries including Spain, Denmark, Norway and the Netherlands announced the withdrawal of staff from their respective embassies.

Canada said the first plane-loads of asylum seekers have already landed in Toronto on Friday, as part of its promise to take in up to 20,000 Afghan refugees, including women leaders and government workers.

On Friday, insurgents took control of four more provincial capitals, having on Thursday seized Kandahar and Herat, the second and third-largest cities. Afghan government forces are in disarray, and there are reports the vice-president, Amrullah Saleh, has fled.

US military intelligence suggests Kabul could come under pressure within 30 days. If trends continue, the Taliban are likely to gain full control of the country in months, it says. Refugees from the insurgents’ relentless offensives elsewhere have flooded the capital.

The refugees Canada will take in include the “particularly vulnerable” still in the country or those who have already fled to neighbouring states. They include human rights activists, persecuted minorities and journalists.

The foreign minister, Marc Garneau, said Canada “owes Afghans a debt of gratitude and we will continue our efforts to bring them to safety”. The immigration minister, Marco Mendicino, said: “The situation in Afghanistan is heartbreaking and Canada will not stand idly by.”

Officials said Canadian special forces formed part of a contingency plans to airlift embassy staff out of Kabul, but details were not provided due to the sensitive nature of the security operation.

For Kabul residents and the tens of thousands who have sought refuge there in recent weeks, the overwhelming mood was one of confusion and fear of what lies ahead. “We don’t know what is going on,” one resident, Khairddin Logari, told Agence France-Presse.

The UN secretary general, Antonio Guterres, said earlier he was “deeply disturbed” by accounts of poor treatment of women in areas seized by the Taliban, who imposed an ultra-austere brand of Islam on Afghanistan during their 1996-2001 rule.

“It is particularly horrifying and heartbreaking to see reports of the hard-won rights of Afghan girls and women being ripped away,” Guterres said.

The scale and speed of the Taliban advance has shocked Afghans and the US-led alliance that poured billions into the country after toppling the Taliban in the wake of the September 11 attacks nearly 20 years ago.

Despite the swift evacuation efforts, the Biden administration said a complete Taliban takeover was not inevitable.

“Kabul is not right now in an imminent threat environment,” Kirby said on Friday, while acknowledging that Taliban fighters were “trying to isolate” the city.

The Taliban offensive has accelerated in recent days, with the capture of Herat in the north and, just hours later, the seizure of Kandahar – the group’s spiritual heartland in the south.

Kandahar resident Abdul Nafi told AFP the city was calm after government forces abandoned it for the sanctuary of military facilities outside, where they were negotiating terms of surrender. “I came out this morning, I saw Taliban white flags in most squares of the city,” he said. “I thought it might be the first day of Eid.”

Pro-Taliban social media accounts have boasted of the vast spoils of war captured by the insurgents – posting photos of armoured vehicles, heavy weapons and even a drone seized by their fighters at abandoned military bases.

In Herat, the Taliban captured long-time strongman Ismail Khan, who helped lead the defence of the provincial capital along with his militia fighters.

Pul-e-Alam, capital of Loghar province, was the latest city to fall on Friday, putting the Taliban within striking distance of Kabul.

With Agence France-Presse