At least 100 oil and gas tankers have been destroyed by fire in a "catastrophe" at Afghanistan's biggest trade crossing with Iran, causing millions of dollars of losses, officials said Sunday.

The huge blaze, which broke out Saturday afternoon at Islam Qala port 120 kilometres (75 miles) from the western city of Herat, has largely been extinguished and an investigation launched into its cause.

"We were told that 100 or 200 tankers have been destroyed, but this number could be higher," Jailani Farhad, spokesman for the governor of Herat province, said after visiting the scene.

During the blaze looters descended on the site, stealing goods that were being imported and exported across the border, Younus Qazi Zada, the head of the Herat Chamber of Commerce said.

"The catastrophe was much bigger than imagined," he said, adding: "Unfortunately, irresponsible people have looted a large number of goods."

Qazi Zada said initial estimates were of "millions of dollars of losses".

Farhad added that investigators needed more time to examine the extent of the losses.

Videos posted on social media on Saturday night showed the towering fire and huge clouds of thick black smoke billowing into the sky.

An AFP photographer at the scene on Sunday said flames and smoke were still rising from the burned-out trucks.

Hundreds of people who say they are the owners of the trucks have gathered at a nearby police cordon, trying to get access.

Around 20 people were injured in the fire, according to Herat health officials.

The finance ministry said early findings suggested the blaze started in a tanker before quickly spreading and causing "heavy financial losses" -- including fuel, tankers and customs facilities.

A delegation from the capital Kabul will investigate.

Damage to power lines from the incident left large parts of Herat province without power on Sunday.

On Sunday, Afghan and Iranian fire services were at the scene extinguishing small remaining blazes.

- Trucks fled over the border -

Islam Qala is one of the major ports in Afghanistan, through which most official trade with Iran is conducted.

Kabul has waivers from Washington allowing it to import oil and gas from Iran despite US sanctions.

Iranian foreign ministry spokesman Saeed Khatibzadeh said the border "was held open for trucks, cars and people running from the fire".

Afghanistan's vice president Amrullah Saleh said hundreds of trucks were allowed to enter Iran to escape the fire.

The Taliban has regularly attacked fuel tankers they suspect of supplying foreign troops in the country.

In 2014, the hardline group destroyed more than 200 fuel trucks on Kabul's outskirts in an attack.

There was no indication that militants were behind Saturday's blaze.

However, insurgents assaulted a nearby security post shortly after the blaze broke out, taking advantage of the situation, Farhad said Saturday. 

Security forces have been deployed around the port area.

Afghanistan has been hit by a surge in violence despite ongoing peace talks between the Taliban and the Afghan government, which have so far failed to achieve a breakthrough.

The rise in violence has led US President Joe Biden's administration to launch a review of a deal signed between Washington and the terror group last year that paved the way for the withdrawal of all American troops in the coming months.