Thick black smoke billows from the explosion site.

A gas tanker exploded on Saturday at a customs post in Afghanistan on the Iranian border, local Afghan officials said, adding that scores of officials were assessing the damage caused by the massive fire.

A video circulating on social media showed thick black smoke billowing from a huge ball of fire.

There were no immediate reports of casualties.

Iranian media quoted an Iranian customs spokesman in the northeastern province of Khorasan as saying the blast had occurred around noon at the Islam Qala border customs post in Afghanistan.

COVID-19 SCARE

On the other hand, Iran's President Hassan Rouhani warned on Saturday of a Covid-19 "fourth wave" as cases rise in certain areas of the Middle Eastern country hardest hit by the pandemic.

"This is a warning for all of us," Rouhani said in televised remarks.

He said some cities in the southwestern province of Khuzestan were now "red" -- the highest on Iran's colour-coded risk level -- after weeks of low alert levels across the country.

"This means the beginning of moving towards the fourth wave. We all have to be vigilant to prevent this," Rouhani added.

The country of more than 80 million people has lost close to 59,000 lives out of more than 1.5 million cases of Covid infection.

Iran has officially registered less than 7,000 daily infections since late December, but the number has crossed this level since early February.