Members of a cell arrested before they could carry out attacks on Bahrain's Interior Ministry and the Saudi Arabian embassy had been in contact with Iran's elite Revolutionary Guards, Bahrain's state news agency BNA reported on Sunday.
Bahrain has often accused Iran of supporting unrest by majority Shi'ites in the Gulf Arab state, which hosts the U.S. Navy's Fifth Fleet.
It said on Saturday the suspects, four of them Bahrainis detained in neighbouring Qatar, had also planned to attack a causeway joining Bahrain to Saudi Arabia. The suspects' interrogation led to the arrest of a fifth member of the group in Bahrain, BNA reported.
Some of the suspects had confessed that the group was set up abroad ... to carry out terror operations in Bahrain ... and was in coordination with the military overseas, including the Revolutionary Guards and the Basij (militia), BNA quoted a Bahraini prosecution spokesman as saying.
A local official in Iran's oil city of Abadan, near the border with Iraq, said authorities detained two Kuwaitis who were in possession of spying equipment, the semi-official Fars news agency reported on Sunday without giving further details.
In March, a Kuwaiti court sentenced two Iranians and a Kuwaiti man to death for being part of an alleged Iranian espionage ring in a case that strained relations between Kuwait and the Islamic Republic.
Kuwaiti media said some of those arrested in the case were gathering information on Kuwaiti and U.S. military sites for the Revolutionary Guards.
Activists among Bahrain's Shi'ite Muslim majority began a series of mainly peaceful demonstrations in the capital in February, pushing for more political freedoms and better living conditions, inspired by uprisings across the Arab world.
The country's Sunni rulers, helped by Saudi security forces, crushed the demonstrations and said the protesters had been supported by predominantly Shi'ite Iran. Tehran has denied involvement in the unrest in Bahrain.
Last month, U.S. authorities said they had broken up a plot by two men linked to Iran's security agencies to assassinate the Saudi ambassador to the United States, Adel al-Jubeir. One of the two was arrested in the United States last month and the other was believed to be in Iran.
Iran has denied the allegations.
(Reporting by Firouz Sedarat In Dubai and Robin Pomeroy in Tehran; Editing by Janet Lawrence)