At least six people died and dozens were injured or missing under rubble after an unfinished high-rise building collapsed in southwestern Iran, officials said.

Choking white dust swirled from where the tower-block had stood, covering surrounding vehicles, as shocked onlookers stared at the wreckage in horror, some screaming.

"Parts of the 10-storey Metropol building, located in Abadan in Khuzestan province, collapsed," state television said. "Six people lost their lives and 27 others were injured in the disaster."

Rescuers clambered over huge slabs of shattered concrete and tangles of twisted metal bars.

The Iranian Red Crescent, in an initial report, said that as many as 80 people could be trapped under the rubble, but hours after the collapse, rescuers lowered the missing to "tens" of people. 

Sniffer dogs were being used to search for them.

"Today is a day of mourning for poor Abadan," residents shouted in a video posted on social media.

Cars squashed flat  

Rescuers were seen on television rushing out some of the wounded who had been pulled from the wreckage on stretchers.

Several cars were crushed by blocks of falling debris -- some apparently with their occupants still inside, according to images.

A crane began work to try to clear some of the debris, including picking up a car squashed flat by the falling tonnes of concrete.

Authorities have not provided further details about the victims.

"About 40 percent of the building has been destroyed, and (the rest) is in danger of completely collapsing," city governor Ehsan Abbaspour said on television, ordering the evacuation of neighbouring buildings.

The rest of the building still standing, with floors sagging, towered over the pile of debris.

Shattered concrete 

State television broadcast live images of the tragedy, showing several teams of firefighters clearing twisted metal.

The building is located on Abadan's busiest street where "commercial, medical and office" buildings predominate, according to state television. 

President Ebrahim Raisi, on a visit Oman, ordered the fire brigade to "immediately deploy rescue forces and equipment necessary for the removal of debris" in Abadan, the official IRNA news agency reported.

The cause of the collapse was under investigation, and the owner of the tower-block and the building contractor have been arrested, city prosecutor Hamid Maranipour said.

Abadan, a city of some 230,000 people, lies 660 kilometres (410 miles) southwest of the capital Tehran.

In January 2017, 22 people died -- including 16 firefighters -- after fire engulfed the Plasco building, a 15-story shopping centre and the oldest high-rise in Iran's capital.