A rocket landed at a residential and operations headquarters of several global major oil companies, including US giant ExxonMobil, near Iraq's southern city of Basra early on Wednesday, wounding two Iraqi workers, police said.

There was no immediate claim of responsibility. It came after two separate attacks in as many days on bases housing US military personnel in Iraq, as tension increases between the United States and Iran.

The rocket hit the Burjesia site west of the city, police said.

The United States evacuated hundreds of diplomatic staff from its Baghdad embassy last month, citing unspecified threats from Iran against US interests in neighbouring Iraq, where Tehran supports some Shi'ite militias.

Wednesday's incident came just as Exxon staff who were also evacuated after the diplomats' departure had begun to return to Basra.

A security source said Exxon was again preparing to evacuate about 20 foreign staff immediately.

Oil officials said operations including exports from southern Iraq were not affected by the incidents.

Other companies operating at the site include Royal Dutch Shell PLC and Italian Eni SpA, the oil officials said. Police said the rocket was a short-range Katyusha missile that landed 100 metres from the part of the site used as a residence and operations centre by Exxon.

Burjesia is near the Zubair oilfield operated by Eni.

Washington has ramped up sanctions pressure on Iran in recent months and says it has sent additional forces to the region over tension with the Islamic Republic.

It blames Tehran for attacks on tankers in the Gulf of Oman last week. Tehran denies it was involved.

Both sides say they do not want war, but analysts warn such incidents could escalate violence in the region.