US Defence Secretary Jim Mattis met Saudi King Salman on Wednesday, beginning a series of talks to reinvigorate the Riyadh-Washington alliance in the face of common adversary Iran.

The retired four-star Marine general, accompanied by a US delegation, was welcomed by Salman at Al-Yamama Palace in Riyadh, telling the monarch: “It's good to be back.” Mattis, who commanded troops during the 2003 invasion of Iraq, arrived in the kingdom on Tuesday afternoon to listen to Saudi leaders and learn “what are their priorities”, an American defence official said earlier.

The United States and Saudi Arabia have a decades-old relationship based on the exchange of American security for Saudi oil.

But ties between Riyadh and Washington became increasingly frayed during the administration of president Barack Obama.

Saudi leaders felt Obama was reluctant to get involved in the civil war in Syria and was tilting toward Riyadh's regional rival Iran.

The Sunni Muslim kingdom “felt marginalised” during international negotiations on a nuclear accord with Shiite Iran, the defence official said.

That deal, signed in July 2015 by the Obama administration, saw the lifting of international sanctions in exchange for guarantees that Tehran will not pursue a nuclear weapons capability.

Mattis was also to meet Crown Prince Mohammed bin Nayef and Salman's son, Deputy Crown Prince Mohammed bin Salman, who holds the defence portfolio.

The deputy crown prince last month held talks with President Donald Trump in Washington.

New Iran sanctions

Saudi leaders worry about Iran interfering in Arab countries by using their communities to advance their pawns, as in Bahrain, Lebanon and Yemen.

Bordering Saudi Arabia, Yemen has been torn for more than two years by a civil war between Houthi rebels, their allies, and pro-government forces aided by a Saudi-led military coalition that receives some military support from the US.

The Saudis have found a more favourable ear in Washington under Trump, who denounces Iran's “harmful influence” in the Middle East.

Mattis has called Iran “the single biggest state sponsor of terrorism in the world”.

In February, Trump imposed new sanctions on Tehran after a ballistic missile test launch, and in response to its support for Yemen's rebels.

The US military is watching Houthi activities along the strategic Bab al-Mandab strait connecting the Red Sea with the Indian Ocean.

Yemeni rebels in late January attacked a Saudi warship in the Red Sea, and they are also believed to have fired missiles towards US warships in the area.

The United States accuses the rebels of deploying coastal defence missiles and other weapons which threaten free navigation in the waters which are vital to global trade.

Tensions with Iran were already high over another waterway, the Strait of Hormuz which links the Gulf and the Indian Ocean.

Anti-IS campaign

Trump's Yemen focus has so far been on a major escalation of attacks against jihadists from Yemen-based Al-Qaeda in the Arabian Peninsula.

Pentagon officials have so far remained cautious on what they might do to support Saudi Arabia.

The Trump administration could, for example, lift a freeze on the delivery of precision guided bombs to the kingdom.

Obama's government blocked their transfer in December because of concerns over civilian casualties in Yemen.

Trump could also decide to reinforce support for the Arab coalition in Yemen, even if his administration wants the return “as quickly as possible” of UN-backed peace talks, as Mattis said at the start of his Saudi trip.

Washington would also like Riyadh to step up its participation in the campaign against the militant Islamic State group in Iraq and Syria.

Options include the kingdom intensifying its aerial operations or its humanitarian aid, the American defence official said, describing the fight against IS as “our number one concern”.

Mattis is expected to evoke with the Saudis Washington's intentions towards Syria, where Riyadh has aided some militant groups.

For the first time in six years of Syria's war, the American military intervened directly against the regime of President Bashar al-Assad by firing 59 cruise missiles at a Syrian government airbase earlier this month.

Saudi Arabia applauded the strike after a suspected chemical bombing of a militant-held town which the US blamed on Assad.

After Saudi Arabia, Mattis travels Thursday to Egypt and then to Israel on Friday before returning to the Gulf Saturday for talks in Qatar.

Some conservative American analysts say the US should build a common front between Israel and the Gulf monarchies, which have no diplomatic relations with the Jewish state.