US Secretary of State John Kerry is visiting Qatar to try to reassure Gulf allies over the nuclear deal with Iran.
The US has argued that the deal, aimed at stopping Iran from making a nuclear weapon in exchange for sanctions relief, will make the region safer.
The Gulf states accuse Iran of stoking unrest in Syria, Iraq, and Yemen.
Mr Kerry is also due to meet foreign ministers from Saudi Arabia and Russia to discuss the conflict involving Islamic State (IS) militants in Syria.
Russia, as well as Iran, is allied with Syrian President Bashar al-Assad, while the Saudi government and the US have called for him to leave office.
However, all sides are opposed to IS, which has emerged as a major player in Syria’s civil war.
On Monday, a Kremlin spokesman criticised US plans to provide air cover for Syrian rebel groups fighting President Assad’s forces, as well as those fighting IS.
“Helping Syrian opposition, let alone helping with financial or technical means, would lead to a further destabilisation of the situation in the country,” Dmitry Peskov told reporters.
The US, working alongside Turkey, is hoping to drive IS from northern Syria by carrying out air strikes to support selected rebel groups.
Mr Kerry is expected to discuss the Iranian nuclear deal with members of the Gulf Co-operation Council – a Sunni-dominated regional body bringing together Saudi Arabia, the United Arab Emirates, Kuwait, Oman, Bahrain and Qatar.
Speaking earlier in Cairo, Mr Kerry acknowledged regional concerns over the nuclear deal – but insisted it would lessen any threat from Iran.
“If Iran is destabilising, it is far, far better to have an Iran that doesn’t have a nuclear weapon than one that does,” he told reporters.
The deal was reached in Vienna in July after marathon talks between Iran and six world powers, including the US and Russia.
The agreement envisages lifting economic sanctions in exchange for curbs on Iranian nuclear activity, which are intended to stop the country from developing a bomb. Iran has always insisted that its nuclear work is peaceful.
Many Sunni Arab countries remain wary of the Shia government in Tehran, accusing it of supporting proxies in conflicts with Sunni forces across the region.