An “unprecedented” number of people are travelling from Afghanistan to Pakistan through the official border crossing, according to local officials, as the carnage in Kabul after the airport suicide attack has driven more to try to flee the country.

While Pakistan has said it will not accept any Afghan refugees, the Spin Boldak-Chaman land border crossing between Pakistan and Afghanistan has remained open, and in recent days hundreds of thousands of Afghans have crossed over.

Only people who are travelling to Pakistan for medical treatment or have proof of residence in the country are allowed to cross, but people smugglers have been assisting families to get over the border.

On Friday, after the suicide attack at Kabul airport that killed more than 100 Afghan civilians and 13 US soldiers, the numbers crowding at the border increased further.

A local health official, who asked not to be named as he was not authorised to speak to the media, said: “There has been an unprecedented rise in the numbers of Afghan people and refugees entering into Pakistan from various provinces of Afghanistan in the last two days. Today, even more people crossed into Chaman than yesterday.”

Another person at the scene said the numbers at the border almost doubled on Friday. Every day, tens of thousands of people have gathered at the border and about 20,000 have been crossing every day, almost triple the usual 6,000. Those fleeing have included Pashtuns, the largest ethnic group in Afghanistan, as well as Tajiks and minorities such as Hazara Shias.

Pakistan is already home to most of the 2.2 million Afghans registered abroad. The UN refugee agency, UNHCR, is gearing up for as many as half a million people or more to flee from Afghanistan in a “worse-case scenario” in the coming months, with Pakistan likely to be where the majority would flee.

About 515,000 refugees have already fled recently and UNHCR said the situation in Afghanistan “remains uncertain and may evolve rapidly”.

While the Pakistan health official referred to many crossing over as refugees, a recognition they were fleeing to Pakistan, officially the government has denied there has been a mass exodus of Afghans illegally entering the country through the border crossing.

“We are not allowing Afghan refugees to enter and cross our border. There might have been a few individual cases but there is no influx,” said Liaquat Shahwani, the spokesperson for the Balochistan state government.

Shahwani said that for those who did cross over, they would be confined to refugee camps being built in bordering areas, and he called on UNHCR and other international agencies to help build and run them.

Balochistan, the region where the border town of Chaman is located, is already one of the most turbulent and war-torn regions of Pakistan, and is subjected to frequent attacks carried out by Baloch insurgent groups and Islamic militants, including Islamic State.

Following the attack on Kabul airport on Thursday, where IS militants detonated a suicide bomb in the crowds, Shahwani said security was being stepped up in the Chaman border area.

“There is no official warning for any terrorist attacks in bordering areas but the security forces on the border are on alert and they are guarding the border,” said Shahwani. “They are well prepared to deal with any situation. In recent days and weeks, there have been some terrorist attacks but the circumstances are under our control in Balochistan.”