A rocket landed at the headquarters for several major global oil companies, including US giant ExxonMobil, in Iraq's southern city of Basra early on Wednesday, wounding three Iraqi workers, police said.
The rocket hit the Burjesia residential and operations headquarters west of Basra, they said.
A security source said Exxon was preparing to evacuate some 20 foreign staff immediately.
Other companies operating at the site include Royal Dutch Shell PLC and Italian Eni SpA, oil officials said.
Police said the rocket was a short-range Katyusha missile that landed 100 metres from the section of the site used as a residence and operations centre by Exxon.
Burjesia is near the Zubair oilfield operated by Eni.
Exxon evacuated staff last month after the United States cited unspecified threats from Iran for a decision to take hundreds of diplomatic staff out of Iraq.
Exxon had begun returning staff to Iraq, however, before Wednesday's incident.
Royal Dutch Shell said on Wednesday that all its staff in Iraq are accounted for and its operations in the country are normal, after a rocket struck the site of headquarters of several foreign oil firms near Iraq’s southern city of Basra.
“We remain vigilant and continue to monitor the security situation and liaise with local authorities,” said a Shell spokesman in a statement to Reuters.