Saudi Aramco emergency crews contained fires at its plants in Abqaiq and Khurais, as a result of terrorist attacks with projectiles, which resulted in production suspension of 5.7 million barrels of crude oil per day (bpd), the company said on its website.

Leading regional experts said this new development was expected to send oil prices soaring and increase tensions in the Middle East.

The pre-dawn strikes follow earlier cross-border attacks on Saudi oil installations and on oil tankers in Gulf waters, but these were the most brazen yet, temporarily crippling much of the nation's production capacity, reported Reuters.

Saudi Arabia is the world's biggest exporter, shipping more than 7 million barrels of oil to global destinations every day, and for years has served as the supplier of last resort to markets.

The US condemned the attacks and Trump told Saudi Crown Prince Mohammed Bin Salman that Washington was ready to work with the kingdom to guarantee its security, according to the White House.

US Secretary of State Mike Pompeo put the blame squarely on Iran, writing on Twitter that there was "no evidence the attacks came from Yemen."

"Amid all the calls for de-escalation, Iran has now launched an unprecedented attack on the world's energy supply," Pompeo said.

Crown Prince told Trump by telephone that Riyadh had the will and capability "to confront and deal with this terrorist aggression," according to Saudi state news agency SPA.

Meeanwhile, the US Department of Energy said it was ready to release oil from its strategic petroleum reserve if necessary.

Energy Secretary Rick Perry also said his department would work with the International Energy Agency, which coordinates energy policies of industrialised nations, if global action is needed

After visiting the incident locations, Amin H Nasser, Saudi Aramco president & CEO, said: “We are gratified that there were no injuries. I would like to thank all teams that responded timely to the incidents and brought the situation under control. Work is underway to restore production and a progress update will be provided in around 48 hours.”

The company will release additional information as it becomes available, the website statement said.