Six U.S.-allied Gulf Arab states have demanded that Iran end what they called interference in the region, reiterating a long-held mistrust of their main rival.
The Islamic Republic denies trying to subvert Saudi Arabia and its wealthy Gulf neighbours.
A communique issued at the end of a two-day summit of the Saudi-led Gulf Cooperation Council (GCC) also urged action to halt mass killings and violations of international law in Syria.
The oil-producing GCC states wield influence out of proportion to their sparse populations due in part to global energy and investment links, generous international aid and Saudi Arabia's role as home to Islam's two holiest sites.
"The council expressed its rejection and condemnation of the continuing Iranian interference in the affairs of the Gulf Co-operation Council's states and called on Iran to stop these policies," the communique said.
On the conflict in Syria, the statement, read out by GCC Secretary-General Abdulatif al-Zayani, added: "We ask the international community for serious and swift moves to stop these massacres and these severe attacks".
Kuwait said it would host an international humanitarian donor conference for Syria in late January, amid concern for millions of Syrians suffering war, homelessness and winter cold.
'Lots of meddling'
Gulf Arab leaders have long called for Syrian President Bashar al-Assad to step down, and in November the GCC recognised a newly-formed opposition coalition as the legitimate representative of the Syrian people.
The communique did not elaborate on Iran, but the most common Gulf Arab complaint about alleged Iranian meddling in the region relates to Bahrain, which has repeatedly accused Tehran of interference in its internal politics.
Iran sees the Gulf as its own backyard and believes it has a legitimate interest in expanding its influence there.
Bahraini Foreign Minister Khalid Bin Ahmed Bin Mohammed Al Khalifa told reporters Iran posed a "very serious threat".
"Politically, (there is) lots of meddling in the affairs of GCC states; an environmental threat to our region from the technology used inside nuclear facilities; and there is of course the looming nuclear programme," he said, referring to Iran's disputed atomic work.
"So the threat level is quite high, but we are ready if faced with circumstances that require action."
While not racked by disturbances on the scale of Syria or Egypt, Bahrain has been volatile since pro-democracy protests led by its Shi'ite Muslim majority erupted last year.
Scattered smoke plumes rose from Bahrain's Sitra and Sanabis districts on Tuesday, apparently cuased by youths burning tyres, but no major demonstrations were reported by activists.
Bahrain's Sunni Muslim rulers brought in Saudi and United Arab Emirates forces last year to help quell the protests, and Shi'ite power Iran condemned the move, saying it could lead to regional instability. Bahrain has accused Iran of being behind the unrest. Tehran denies this.
GCC force long seen as ineffective
Bahrain's Shi'ites say they are marginalised politically and economically, a charge the government denies. It has rejected the protesters' main demand for an elected government.
The summit statement said the GCC would set up a unified military command to tighten defence cooperation but offered few details of a project long prey to sensitivities about sovereignty. Security in the waterway, through which 40 percent of the world's seaborne oil exports passes, has been dominated for decades by the United States.
But uprisings against long-standing governments across the Middle East and rivalry with Iran over the conflict in Syria have stirred calls among Gulf Arabs to speed up long-stymied efforts to integrate their own foreign and security policies.
The GCC said it had "supported the creation of a unified military command that organizes and plans and leads the ground, naval and airforces."
The communique did not elaborate. But Mustafa Alani, a security analyst, told Reuters that he understood the idea was to have a standing command rather than a functioning one, and that it would only operate in times of crisis.
The GCC already has a pan-GCC military force - the 9,000-strong Peninsula Shield, created in 1986 and based in Saudi Arabia. It took part in the 1991 Gulf war and was deployed in Kuwait during the U.S.-led invasion of Iraq in 2003.
But the Saudi-based force is widely seen as ineffective. Gulf Arab states have faced a host of obstacles to military integration, including a lack of common equipment, their own reliance on their U.S. ally and concern among some states about potential Saudi dominance of any joint military effort.