The attacks early Saturday targeted two key oil installations, causing massive fires and taking out half of the kingdom's vast oil output.

The operation was claimed by Iran-backed Huthi rebels in Yemen, where a Saudi-led coalition is bogged down in a five-year war.

But the Wall Street Journal has reported that officials were investigating the possibility the attacks involved missiles launched from Iraq or Iran.

Prime Minister Adel Abdel Mahdi on Sunday denied reports Iraqi territory "was used for drone attacks on Saudi oil facilities".

"Iraq is constitutionally committed to preventing any use of its soil to attack its neighbours," he said in a statement.

"The Iraqi government will be extremely firm with whomever tries to violate the constitution."

Iraq is home to several Iran-backed militias and paramilitary factions, placing it in an awkward situation amid rising tensions between its two main sponsors, Tehran and Washington.

United States Secretary of State Mike Pompeo squarely accused Tehran of being behind Saturday's operation, saying there was no evidence the "unprecedented attack on the world's energy supply" was launched from Yemen.

Iraq has called for its territory to be spared any spillover in the standoff between the US and Iran, which has included a series of attacks on shipping in sensitive Gulf waters.

Recent raids on bases belonging to Iraqi Shiite paramilitary groups linked with Iran, attributed to Israel, sparked fears of an escalation.

There have been no military consequences so far, but the strikes have heightened divisions between pro-Tehran and pro-Washington factions in Iraq's political class.

Baghdad has recently moved to repair ties with Saudi Arabia, a key US ally -- much to Iran's chagrin.

Riyadh recently announced a major border post on the Iraqi frontier would reopen mid October, after being closed for almost three decades.