The number of people killed in the explosion that rocked a major port in southern Iran has risen to 40 as of Sunday, according to Iranian state news agency IRNA.

More than 700 people were injured in the blast, according to the report, and firefighters were still at the scene to extinguish the fire, but were nearly done with 80% as of Sunday, state TV quoted Interior Minister Eskandar Momeni as saying.

The port, located in Hormozgan province more than 1,000 km (620 miles) south of the capital, Tehran, is Iran's largest commercial port. 

Local media have reported that the port handles around half of Iran's total trade and around 85-90% of all container traffic. 

State TV added that the port had resumed some operations.

What happened at the port?

Saturday's explosion was so powerful that people heard and felt it up to 50 kilometers (30 miles) away, the state-run Fars news agency reported.

According to the semi-official Tasnim news agency, the shock wave from the explosion caused severe damage to most buildings at the port.   

State TV said hundreds of casualties had been taken to "nearby medical centers."

It also reported that schools and offices in the nearby provincial capital of Bandar Abbas had been ordered to remain closed on Sunday amid high levels of air pollution from chemicals such as ammonia, sulfur dioxide and nitrogen dioxide.

The Health Ministry has urged residents to avoid going outside "until further notice" and to use protective masks.

Saturday is the start of the working week in Iran, which means the port was likely filled with workers when the blast occurred. 

Explosion may have been caused by chemicals stored at port, initial reports say

Although authorities have not announced any cause of the explosion, various initial reports point to the possible involvement of inflammable chemicals stored at the port.

State TV carried a statement by the port's customs office saying that the blast may have been ignited by a fire in the hazardous and chemical materials storage depot.

The New York Times quoted a source with ties to Iran's Islamic Revolutionary Guard Corps, who spoke on condition of anonymity, as saying sodium perchlorate — a key ingredient in solid fuel for missiles — could have been what blew up.

The Associated Press news agency reported that the blast may have occurred because the chemical may have been improperly stored.

There is no other confirmation about the kind of chemical that may have set off the blast.

Iran's defense ministry says no military cargo at site

Iran's Defense Ministry said on Sunday there was no military cargo at the blast site. 

A ministry spokesman described reports that missile fuel had been imported through the port as "baseless."

However, Germany-based Iran expert Ali Fathollah-Nejad told DW that's not likely to be the case.

"There's a strong indication that the explosions were due to chemicals that were shipped to Iran from China to be used for solid fuels for ballistic missiles," he said.

The blast took place on Saturday, when Iranian and US delegations were meeting in Oman for a third round of high-level talks on Tehran's nuclear program and with both sides having reported progress.

Edited by: Roshni Majumdar and Kieran Burke