The US secretary of state, John Kerry, has met Gulf state ministers to ease their concerns about warming US relations with Iran and seek agreement on which Syrian opposition groups should be represented at upcoming peace talks.

Speaking after a meeting in Riyadh with foreign ministers from the Gulf Cooperation Council (GCC), which has sided with Saudi Arabia in its spat with Iran, Kerry and the Saudi foreign minister, Adel al-Jubeir, presented a united front.

They said the US and the council agreed on the need to confront destabilising Iranian activities in the region and on an unspecified understanding that will allow the UN-led Syria talks to begin next week as planned.

Kerry said: “Let me assure everybody that the relationship between the United States and the GCC nations is one that is built on mutual interest, on mutual defence and I think there is no doubt whatsoever in the minds of the countries that make up the GCC that the United States will stand with them against any external threat.”

Jubeir denounced Iran for its “hostile and aggressive stance” against Arab nations, but said he did not believe Washington would act rashly in dealing with Tehran because of the nuclear deal, which was implemented earlier this month and has given it access to billions in formerly frozen assets.

“Overall, the United States is very aware of the mischief that Iran’s nefarious activities can do in the region,” Jubeir said. “I don’t believe the United States is under any illusion as to what type of government Iran is.”

He criticised Iran for briefly taking 10 US sailors captive in early January, saying “normal countries do not act like this”.

The Saudi minister also took a swipe at Tehran by noting that in the prisoner swap that included the release of four imprisoned Americans in Iran, none of the seven Iranians cleared of charges in the US had opted to return to Iran. It “tells you what a great country Iran is that no one wanted to return to it”, he said.

Kerry avoided such blunt criticism, but stressed that the US shared concerns about Iran’s behaviour and would act against it if necessary, including imposing new sanctions, as it did last week in response to Iranian ballistic missile tests.

Shia-led Iran and Sunni-ruled Saudi Arabia are fierce regional rivals that support opposite sides in the wars in Syria and Yemen. Relations plunged to a new low when Saudi Arabia executed a prominent Shia opposition cleric earlier this month, sparking outrage from Shias across the region and igniting mob attacks on Saudi diplomatic missions in Iran.

Saudi Arabia and some of its allies responded to those attacks by cutting diplomatic ties with Tehran, and accused Iran of being behind numerous terrorist attacks around the world over the past three decades.

Although both Riyadh and Tehran have said the mutual animosity will not affect the Syria talks, there are still serious disagreements over who can represent the opposition at the negotiations. They were due to begin on Monday but are likely to be delayed for several days.

Kerry, who also met Saudi Arabia’s King Salman, the deputy crown prince and the chief negotiator for the Saudi-backed Syrian opposition, said that despite ongoing disagreements over who should attend, an understanding had been reached on how to begin the talks.

He did not elaborate but said the more than 20 nations and groups that make up the international Syria support group would meet almost immediately after the first round of peace negotiations to address any issues that arise in Geneva.

“We are quite confident that there is a way to invite the various interested stakeholders that provides for cohesion and the ability to make the process move forward,” Kerry said.

He also spoke to the Russian foreign minister, Sergei Lavrov, on Saturday.

“Particular attention was paid to the need to form a truly representative delegation of the opposition and ensure compliance with the requirements of the negotiating agenda of the UN security council resolution 2254, including the fight against Isis and other terrorist groups, as well as respect for the right of Syrians themselves to determine the fate of their country,” the Russian foreign ministry said.

Russia is a close ally of the Syrian government, and began carrying out airstrikes in Syria last year. Moscow says it is targeting Isis and other extremists, but the airstrikes have also hit western-backed rebels.

Kerry is in Saudi Arabia on the second leg of his latest round-the-world diplomatic mission, which began in Switzerland and will also take him to Laos, Cambodia and China.