London (CNN Business) Oil workers from the United States are leaving Iraq after Iranian military leader Qasem Soleimani was killed in a US airstrike in Baghdad.

The Iraqi oil ministry said in a statement Friday that "a number" of Americans working in southern Iraq were leaving the country after the United States urged its citizens to depart immediately because of heightened tension in Iraq and the region.

Other foreign workers were not departing and oil fields across the country were operating normally, the Iraqi oil ministry said.

Oil prices moved higher Friday as investors reacted to the risk that the death of Soleimani could prompt retaliation by Iran, including attacks on US assets in the region

Exxon Mobil, a major US oil company, has operations in southern Iraq. Britain's BP and Anglo-Dutch companyalso work in the region.

In a statement, Exxon said it was "closely monitoring the situation" and that it "has programs and measures in place to provide security to protect its people, operations and facilities."

"We are committed to ensuring the safety of our employees and contractors at all of our facilities around the world," the company added. It declined to comment on staffing at its facilities in Iraq.

BP BP would not comment on staff movements or whether it was ramping up security measures at its facilities. Shell, which owns a stake in the Basrah Gas Company, declined to comment.

Exxon XOM is the lead contractor in a project to redevelop the West Qurna I oil field in southern Iraq. Indonesian, Chinese and Iraqi companies are also involved in the project, according to Exxon's website. Exxon said in its statement that production at West Qurna I was continuing normally. A Shell affiliate exited the project in 2018.

Exxon also has a presence in Baghdad and the Kurdistan region of Iraq, according to its website.

BP has operations in the giant Rumaila oil field in southern Iraq, in partnership with Chinese and Iraqi companies. BP estimates that the field has around 17 billion barrels of recoverable oil.

Barham Salih, the president of Iraq, has called for restraint following the attack, while Iran's Supreme Leader Ayatollah Ali Khamenei has vowed "harsh revenge."

Iraqi oil production has recovered strongly in recent years, hitting 4.7 million barrels per day in late 2019, according to the International Energy Agency.

— Charles Riley, Sara Mazloumsaki and Kareem Khadder contributed reporting.