West Qurna-2, which is being developed by Russia's Lukoil, is not yet producing oil but workers at the site said construction of a major crude oil processing plant had been disrupted by the protest.

Officials of the state-run Southern Oil Company said there was no disturbance around any of the producing oilfields in the south of Iraq and production was proceeding as normal.

Iraq's oil ministry said the West Qurna-2 incident would lead to a rethink of security at the field, which is a key element in Iraq's drive to become one of the world's largest oil exporters.

Iraqi oil officials expect the field's output to reach 140,000 barrels per day (bpd) by the end of 2024.

An Iraqi oil police official said the force called in the army to restore order after the protesters stormed offices and smashed surveillance cameras at the oilfield.

"At least 400 angry protesters breached the main gate and broke into West Qurna oilfield," an official at West Qurna Phase 2 told Reuters. "Most of the foreign and local staff left the field as a safety measure and work was significantly affected."

Police sources said the protesters trashed the offices of an Iraqi company hired by Samsung Engineering before trying to break into the South Korean builder's headquarters, throwing stones at the main gate.

"Protesters were on a rampage. They were asking for jobs. We got scared and left and escaped to other headquarters in Basra," a West Qurna worker told Reuters.

Samsung has a major contract with Lukoil to build crude facilities at West Qurna-2, which is the world's second-largest undeveloped field with recoverable oil reserves of around 14 billion barrels, according to Lukoil.

The Samsung contract, worth $998 million, involves construction of an oil export pipeline, a tank farm, a power station and a gas processing plant, an oil gathering system, central processing facilities and a water supply system.

"We will re-consider security measures around the West Qurna oilfield after this unfortunate incident," an Iraq Oil Ministry official said.

(Reporting by Aref Mohammed; writing by Ahmed Rasheed and Isabel Coles; editing by Keiron Henderson and Anthony Barker)