Shafaq News/ At least four people were killed and four wounded in an explosion in Iraq's southern oil hub Basra Tuesday, a source said.

It was not immediately clear what caused the explosion, which sent a column of black smoke into the sky. Local news reports initially reported a car bomb, but a source told Shafaq News Agency that a motorcycle had exploded.

It was not immediately clear whether a bomb had been rigged to the motorcycle or if it was a suicide bombing.

"According to preliminary information, highly explosive material was rigged to the motorcycle which exploded near a vehicle, burning it completely and damaging other vehicles nearby," the source said.

The explosion took place at al-Somood Bridge near the Chancellery building of the Basra Teaching Hospital.

The Security Media Cell (SMC) confirmed that the blast resulted from a booby-trapped motorcycle.

"The incident led to the martyrdom of four citizens. Four others were injured from the flames that caught nearby vehicles," SMC said, "forensic experts and technicians are investigating the site to reveal the circumstances of the explosion. The Cell will announce further details in a report later."

Explosions have been rare in Basra in recent years, particularly since the defeat of the Sunni militant Islamic State group in 2017.

The oil-rich Basra is predominantly Shiite. No one immediately claimed responsibility for the explosion. There was no immediate comment from Iraqi officials in Baghdad.