JEDDAH - Saudi Arabia's leaders sought assurances on Wednesday that the United States stands firmly against Iranian "interference" in the Middle East, during a visit by Defence Secretary Ashton Carter.

Carter arrived as part of a regional tour trying to allay the concerns of US allies that Iran could still be able to develop an atomic weapon despite an agreement reached this month between Tehran and six major powers led by Washington.

The US defence secretary met King Salman, then held talks with the king's powerful son Deputy Crown Prince Mohammed bin Salman, who is defence minister and second-in-line to the throne. Gulf states are wary of the overtures to Tehran by Washington, their traditional defence partner.

Riyadh and its neighbours believe the deal will only embolden their regional rival, whom they accuse of meddling in Iraq, Syria, Lebanon and Yemen.

Anwar Eshki, chairman of the Jeddah-based Centre for Strategic and Legal Studies, said he believed Carter would try to "reassure the Gulf countries, and the kingdom especially, that the US will not allow Iran to carry out activities that will destabilise the Middle East."

He said Saudi Arabia would talk about boosting its defences and "how to confront Iran" if destabilising actions increase as a result of the nuclear deal.

The agreement, which ended a 13-year standoff, requires Iran to curb its nuclear capabilities including the number of uranium centrifuges. International monitors will supervise the process, which in exchange will reduce sanctions that have crippled Iran's economy. Saudi Arabia and Israel are "on the same page" with their concerns about the agreement, a Western diplomatic source said.

The deal would see Iran's oil exports gradually resume and billions of dollars in frozen assets unblocked. Israeli Prime Minister Benjamin Netanyahu, who Carter met on Tuesday, has voiced fears the nuclear deal will help fund Iranian "aggression."

The Saudis also "think it's a mistake," although "they don't say it as loud and as publicly as the Israelis," said the source. There are worries the Iran deal could spark a nuclear race in the Middle East.

Foreign Minister Adel al-Jubeir last week warned Iran not to use the economic benefits of the nuclear agreement to fund "adventures in the region." If it does, "we're committed to confront it resolutely," said Jubeir, whose country is the world's biggest oil exporter.

In June, France and Saudi Arabia announced a feasibility study for building two nuclear reactors in the kingdom. Saudi Arabia has also reached accords this year with Russia and South Korea on the peaceful use of nuclear energy.

Along with its own nuclear projects, Riyadh is building alliances beyond its ties with Washington in order to counter Tehran, under a more assertive foreign policy adopted since King Salman acceded to the throne in January.

Two months later, the kingdom organised an Arab coalition to conduct air strikes against Iran-backed rebels seizing territory in neighbouring Yemen.

The US has provided aerial refuelling, intelligence and other assistance to that Saudi-led alliance. "There was a difficult situation in Yemen," Salman told Carter, apologising that he could not attend a May summit with President Barack Obama in Washington. Since late last year, Saudi Arabia has also been part of a US-led coalition bombing the Islamic State group which has occupied large parts of Iraq and Syria.

The militant group has committed widespread atrocities there and inspired attacks around the world, including in Saudi Arabia. Carter is trying to respond to regional concerns about Iran by proposing intensified military cooperation with its longtime allies. With the Saudis, there will be talk of training special forces, cyber security, anti-missile defence and other issues, a US defence official said.

Carter is to return mid-afternoon to Jordan, another member of the anti-IS coalition. On Tuesday, he told coalition military personnel at a Jordanian air base that the US and Israel had a "common commitment to countering Iranian malign influence in the region".