US Defence Secretary Ashton Carter landed in Saudi Arabia on Wednesday as part of a regional tour aimed at reassuring Washington's allies over a nuclear deal with Iran.
Carter is to hold talks with King Salman in the Red Sea city of Jeddah, where the kingdom's rulers base themselves during the hot summer months.
He was then to meet for about one hour with Salman's son Deputy Crown Prince Mohammed bin Salman, who is defence minister and second-in-line to the throne.
Carter, who flew in from Jordan, is to return to Amman mid-afternoon.
On Tuesday Carter visited Israel for talks with Prime Minister Benjamin Netanyahu who has predicted the Iran nuclear agreement will lead to a regional nuclear arms race.
Netanyahu also fears the deal will help fund Iranian "aggression".
Sunni-dominated Saudi Arabia, a regional rival of Shiite Iran, worries Tehran could still be able to develop an atomic bomb despite the limitations on its nuclear programme agreed to this month under the deal with Washington and five other major powers.
Like Israel, Riyadh and its Gulf neighbours believe the pact will only embolden Tehran's leaders, whom they accuse of "interference" in Iraq, Syria, Lebanon and Yemen.
In an effort to calm these worries, Carter proposes to intensify military cooperation with Washington's traditional allies in the Middle East.
With the Saudis, there will be talk of training special forces, cyber security, anti-missile defence and freedom of navigation in the strategic Red Sea and Gulf waterways, an American defence official said.
While the Saudis and others are focused on the Iranian influence, Carter has also highlighted the fight against the Islamic State group of jihadists, who have seized large parts of Iraq and Syria.
The organisation has committed widespread atrocities there and inspired attacks around the world, including in Saudi Arabia.
"We will continue to work with Israel and other partners in this region to counter the danger from Iran, even as we do the same with respect to ISIL," Carter said, using another acronym for IS when he spoke on Tuesday in Jordan.
Carter addressed military personnel who are part of a US-led coalition which since last year has been bombing IS.
Saudi Arabia and Jordan belong to that coalition.
Since late March, Riyadh has led a separate Arab coalition bombing Iranian-backed Shiite rebels in neighbouring Yemen.
The US has provided aerial refuelling, intelligence and other assistance to that Saudi-led alliance.