NATO Chief Jens Stoltenberg also said Iran was "destabilising the whole region".

Earlier on Monday, the US released satellite images showing damage from the weekend's "unprecedented" strikes, which it has pinned on Iran.

Iran denies involvement, with President Hassan Rouhani calling the attack a reciprocal act by the "Yemeni people".

Yemen's Houthi rebels - who are aligned with Iran - have claimed responsibility.

However, the US has cast doubt on their ability to carry out strikes of this magnitude and accuracy without assistance.

The Saudi-led military coalition in Yemen, which is in direct conflict with the Houthis, believes Iran provided the weapons.

"We call on all parties to prevent any such attacks occurring again because that can have negative consequences for the whole region, and we are also extremely concerned about a risk of escalation," Mr Stoltenberg told the AFP news agency in an interview.

How unusual was this attack?

The Houthis have launched attacks on Saudi soil before, including on oil pipelines.

But this attack was on a much bigger scale, hitting the world's biggest oil-processing plant and another oil field.

The knock-on effect was a 5% cut in global oil supplies and soaring prices.

Experts say it could take weeks before the facilities are fully functioning again.

What is the US saying?

US President Donald Trump, who over the weekend stopped short of directly accusing Iran, said on Monday that it looked like it was behind the attack.

But he added: "We want to find definitively who did this."

He also said the US was "more prepared than anyone" for conflict, but would rather avoid it.

In a series of tweets, US Defence Secretary Mark Esper accused Iran of undermining "the international rules-based order".

Iran has discounted the possibility of a meeting between President Rouhani and President Trump on the sidelines of the United Nations General Assembly next week.

There had previously been speculation that they could meet to defuse tensions.

How have others reacted?

UN Yemen envoy Martin Griffiths told the Security Council on Monday it was "not entirely clear" who was behind the strike but that it had increased the chances of a regional conflict.

In the UK, Foreign Secretary Dominic Raab also highlighted the uncertainty, while calling the act a "wanton violation of international law"

China and the European Union have, separately, urged restraint.

What happened to oil markets?

The oil price saw its biggest one-day rise since the 1991 Gulf War, soaring 20% but falling back later.

The international benchmark used by traders, Brent crude, jumped to $71.95 (£57.53) a barrel at one point.

Prices eased after President Trump authorised a possible release of US reserves.

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US Energy Secretary Rick Perry told business channel CNBC that it was too early to tell if this would be necessary.

There are concerns that higher prices could continue if tensions worsen further. Mr Trump has tried to downplay the impact on the market.

How did the attacks unfold?

The attacks targeted Abqaiq, the site of the world's largest oil processing plant, run by the Saudi state oil company, Aramco, and the Khurais oilfield.

Khurais is the closest of the targets to the Yemen border, but is still a considerable 770km (480 miles) away.

US officials said there were 19 points of impact on the targets, which could have come from a mix of drones and cruise missiles.

They have told media outlets that they believe the attacks did not originate from Houthi-controlled territory in Yemen, which lies to the south-west of the Saudi oil facilities, and instead were launched from the north or north-west.

The officials said that could suggest launch sites in the northern Gulf, Iran or Iraq but no conclusive evidence has been provided.

Iraq denied at the weekend that the attacks were launched from its territory. Prime Minister Adel Abdul Mahdi said the US assured him in a phone call on Monday that it backed Iraq's position.

Why might the Houthis have attacked Saudi Arabia?

The Houthis have repeatedly launched rockets, missiles and drones at populated areas in Saudi Arabia. The attacks have left at least four civilians dead.

The Yemen conflict escalated in March 2015, when the Houthis seized control of much of the west of the country and forced President Abdrabbuh Mansour Hadi to flee abroad.

Saudi Arabia and allies then began an air campaign aimed at restoring Mr Hadi's government.

The UN says the conflict has claimed the lives of at least 7,290 civilians and left 80% of the population - 24 million people - in need of humanitarian assistance or protection, including 10 million who rely on food aid to survive.