Saudi Arabia and the US have carried out strikes against Iran-backed militias in Iraq, in a potential escalation of the Middle East conflict.

US Central Command (Centcom) said the strikes were against "Iran-aligned terrorists that the Islamic Revolutionary Guard Corps (IRGC) directed to attack US forces and Saudi energy infrastructure".

Iraq's paramilitary Popular Mobilisation Forces (PMF), which is dominated by Iran-backed Shia militias, said at least 20 of its members had been killed in US-Saudi strikes on its bases.

It came hours after Centcom said Iran had launched missiles at US forces in the Middle East in an "attempted surprise attack", ending a lull in hostilities.

The US said the Iranian attack occurred at 17:45 EDT (21:45 GMT) on Tuesday, with all missiles "successfully intercepted".

The IRGC said the missiles had targeted a US air base and a command centre in Jordan "in response to the acts of aggression of the American child-killing army".

It also said that naval forces had struck three oil tankers in the Strait of Hormuz after they "ignored warnings" and sailed along what it described as an "unsafe and illegal route" through the vital Gulf waterway.

Centcom said US and Saudi fighter jets had struck "multiple terrorist logistics and weapons sites across eastern Iraq in a strong response to over 30 IRGC-directed aerial drone attacks in the last 72 hours".

It called the drone attacks "unwarranted" and added that they had failed.

"The IRGC and its terrorist proxies must cease these attacks to avoid further U.S. military response," the Centcom statement said.

The PMF said in a statement on Wednesday morning that at least 20 of its fighters had been killed and 32 wounded in US-Saudi strikes on several of its bases in Iraq.

It called the strikes a "dangerous escalation" as well as a violation of Iraq's sovereignty.

The PMF is an umbrella organisation of mostly Shia Muslim militias that has been formally integrated into the Iraqi armed forces. It includes several Iran-backed groups which are part of a network of proxies that Tehran has used to advance its interests across the Middle East for years.

Those militias have attacked US personnel and interests in Iraq on a number of occasions, aiming to force the remaining US troops to leave the country. The US has regularly struck back at the militias.

An Iranian defence official was quoted by state TV as denying any Iranian connection to the drone attacks on Saudi Arabia, according to Reuters news agency.

The official warned that attributing attacks on US interests in the region to Iran was a "major miscalculation".

On Tuesday, Saudi Arabia said its military had intercepted and destroyed drones targeting petroleum facilities in the country's eastern region.

Defence ministry spokesman Maj Gen Turki al-Maliki alleged the drones were launched by Iran-affiliated militia groups in Iraqi territory.

The reignited hostilities came about 24 hours after US President Donald Trump spoke of "very friendly negotiations" with Iran. At that point, the two sides had not exchanged strikes for a third consecutive day.

The pause in fighting had followed 13 straight nights of bombing by the US, which said it had destroyed tunnels and bridges in southern Iran. Scores of people were reported to have died in the strikes.

Access and control of the Strait of Hormuz remains a key sticking point in talks, which Tehran has denied are even happening.

Oil prices rose early on Wednesday over concerns that an escalation in the conflict could further disrupt energy shipments through the strait. (BBC)