US President Donald Trump has approved the deployment of American troops to bolster Saudi Arabia's air defences after the biggest-ever attack on the kingdom's oil facilities, which Washington has squarely blamed on Iran.

Key points: The Pentagon said military deployment would be primarily defensive in nature

The Pentagon said military deployment would be primarily defensive in nature A total of 25 drones and missiles were launched at two Saudi oil plants last weekend

A total of 25 drones and missiles were launched at two Saudi oil plants last weekend The deployment could further aggravate Iran, which denies responsibility for the attacks

The Pentagon said the deployment, approved on Friday (local time), was "in response to the kingdom's request" and would be primarily defensive in nature, involving a moderate number of US military personnel.

Officials said the US deployment would likely be in the hundreds and the defensive equipment heading to the Middle East would probably include Patriot missile batteries and possibly enhanced radars.

Reuters has previously reported that the Pentagon was considering sending anti-missile batteries, drones and more fighter jets.

The United States is also considering keeping an aircraft carrier in the region indefinitely.

'Strong person's approach' is restraint: Trump

In a joint press conference with Scott Morrison (left), Mr Trump (right) told reporters he wanted to avoid a war with Iran. ( AP: Patrick Semansky )

The announcement reflected Mr Trump's earlier comments when he told reporters that showing restraint "shows far more strength" than launching military strikes, and that he wanted to avoid an all-out war with Iran.

Instead, he laid out new sanctions on the Iranian central bank.

"I think the strong person's approach, and the thing that does show strength, would be showing a little bit of restraint," Mr Trump told reporters during a meeting with Prime Minister Scott Morrison.

"Much easier to do it the other way, and Iran knows that if they misbehave they are on borrowed time."

The US released images claiming to show the aftermath of drone attacks on Saudi oil infrastructure on September 15. ( AP: DigitalGlobe/US Government )

The emphasis on restraint was echoed by US Defense Secretary Mark Esper, who said the US's new deployments in the Gulf were "defensive in nature".

"We will also work to accelerate the delivery of military equipment to the kingdom of Saudi Arabia and the UAE to enhance their ability to defend themselves," he said.

Mr Esper added that the United Arab Emirates, a close ally that hosts US forces, had also requested assistance.

The Pentagon's late Friday announcement appeared to close the door to any imminent decision to wage retaliatory strikes against Iran following the attack, which rattled global markets and exposed major gaps in Saudi Arabia's air defences.

But the deployment could further aggravate Iran, which has responded to previous US troop deployments this year with apprehension.

It denies responsibility for the attack on Saudi Arabia as well as attacks on oil tankers earlier this year.

Yemen's Iran-aligned Houthi movement, which has been battling a Saudi-led military coalition that includes the UAE, has claimed responsibility for the strikes.

US officials point finger at Iran

Remains of what was described as a misfired Iranian cruise missile used in an attack that targeted the heart of Saudi Arabia's oil industry. ( AP: Amr Nabil )

US officials, speaking on condition of anonymity, have pointed to southwest Iran as the staging ground for the attack, an assessment based at least in part on still-classified imagery showing Iran appearing to prepare an aerial strike.

They have dismissed Houthi claims that the attacks originated in Yemen.

One of the officials told Reuters the strike may have been authorised by Iran's Supreme Leader Ayatollah Ali Khamenei.

The United States, wary of getting dragged into another conflict in the Middle East, has troops positioned in Syria and Iraq, two countries where Iranian influence is strong and Iran-backed forces operate openly.

Saudi Arabia has said it was attacked by a total of 25 drones and missiles, including Iranian Delta Wing unmanned aerial vehicles (UAV) and "Ya Ali" cruise missiles.

US Marine General Joseph Dunford, chairman of the Joint Chiefs of Staff, said officials were still hammering out the best array of capabilities to defend Saudi Arabia, noting the difficulty combating a swarm of drones.

"No single system is going to be able to defend against a threat like that, but a layered system of defensive capabilities would mitigate the risk of swarms of drones or other attacks that may come from Iran," General Dunford said.

Reuters/ AP