Bahrain said on Thursday it had seized explosives and bomb-making materials earmarked for use in Bahrain and neighbouring Saudi Arabia in what it described as an Iranian attempt to use Bahraini borders as a base for attacking targets in the region.
Police chief Major-General Tariq al-Hasan said the methods used to assemble the explosives bore "clear similarities" to those of what he called proxy groups of Iran's Islamic Revolutionary Guards Corp operating in Iraq and Afghanistan.
Bahrain, home to the US Navy's Fifth Fleet, often accuses Iran of seeking to subvert the Gulf Arab state.
Iran denies interfering in Bahrain, although it acknowledges it does support opposition groups seeking greater political and economic rights for Bahrain's Shia community.
There was no immediate Iranian comment on Hasan's statement. On Tuesday, Iran's foreign ministry had called for political dialogue in Bahrain and said Manama's "security approach" would not solve the country's political tensions.
Hasan said the seized items, including powerful explosive C4, commercial detonators, advanced circuitry, chemicals and mobile phones, "represent a significant escalation in attempts to smuggle explosives material into Bahrain".
The explosives and bomb-making materials were found in a hidden area behind the wall of a warehouse in the village of Dar Kulaib near Manama, the official Bahrain News Agency said.
It said the location of the warehouse raided on June 6 was revealed by a man identified as Mohammed Jaffar Abdullah, who it said was working on behalf of a cell formed by Iran's Revolutionary Guards to carry out attacks.
Abdullah, it said, was connected to two Bahraini men detained by Saudi Arabia last month for smuggling a similar kind of explosives across the Gulf causeway from Bahrain. It did not make clear whether Abdullah was also now in custody.
The leaders of the cell, Murtadha Majeed Ramadhan Al Sindi, 32, and Qassim Abdullah Ali, 26, are fugitives in Iran wanted for previous offences, the statement said.
Hasan added that the materials confiscated on June 6, and in past such seizures, had been made with "professionalism", indicating international support and sponsorship.
"Of greater concern, however, is the fact that these sophisticated bomb-making materials were destined for Saudi Arabia, a sign that extremists are increasingly using Bahrain's borders as a launch pad for terrorists seeking to carry out attacks elsewhere in the region."
Two suicide bombings last month targeted Shia mosques in Saudi Arabia's Eastern Province, close to the Gulf causeway linking the country to Bahrain, and were claimed by the Islamic State group. The attacks sought to exploit sectarian tensions in the country.
Hasan said the explosive materials found on the two Bahraini men arrested last month matched explosives discovered on a boat travelling in December 2013 to Bahrain from Iraq, where Manama has said Bahraini anti-government activists are trained.