Iran has called on its people to stage rallies after this week's Friday prayers to protest against what it described as a US plan to annex Bahrain to Saudi Arabia.
The Islamic Propagation Coordination Council, which organizes state-backed protests, urged Iranians "to protest against the American plan to annex Bahrain to Saudi Arabia and express their anger against the lackey regimes of Al-Khalifa and Al-Saud."
Leaders of the Gulf Cooperation Council (GCC) discussed on Monday plans to turn the bloc into a union, starting with Saudi Arabia and Bahrain.
"This dangerous plot is the result of the American-Zionist-Britain evil triangle to prevent popular uprisings spreading into other countries of the region and to control the internal crisis in Bahrain which has been caused by the inability of the Al-Khalifa regime to control the situation," the council said on its website.
"Al-Saud and Al-Khalifa should be aware that with this kind of plot they will not stop the popular movement in Bahrain and the movement of Islamic awakening in the region," it added.
The announcement comes after Tehran warned Riyadh's plans to form a union with Manama would deepen the crisis in Bahrain. Saudi Arabia had earlier told Iran to keep out of its relations with the tiny Gulf state.
"Any kind of foreign intervention or non-normative plans without respecting people's vote will only deepen the already existing wounds," Iranian foreign ministry spokesman Ramin Mehmanparast said.
Iranian MPs on Monday condemned the planned union between the two Gulf countries.
"Bahraini and Saudi rulers must understand that this unwise decision will only strengthen the Bahraini people's resolve against the forces of occupation," they said in a letter, referring to the Saudi-led forces.
Bahrain on Tuesday hit out at Iran for interfering in its affairs.
"These statements represent a flagrant interference in the internal affairs of the kingdom and an attack on its sovereignty," the foreign ministry said in a letter of protest handed to the Islamic republic's charge d'affairs, according to BNA state news agency.
Saudi-led Gulf forces rolled into Sunni-ruled Bahrain in March 2011 to boost the kingdom's security forces which a day later crushed month-old, Shia-dominated protests.
Shia-dominated Iran has repeatedly voiced support for the protests in Bahrain and strongly condemned the deployment of Saudi-led forces.
The GCC was formed in 1981 as the Sunni-dominated monarchies of the Gulf aimed to bolster security after the 1979 Islamic revolution in Iran that was followed by an eight-year war between Baghdad and Tehran.
The Bahrain issue is sensitive one to Iran, where a nationalist-conservative movement within the Islamic regime still considers the island, controlled by Persia before being colonized by Britain and then to gain independence in 1971, as an Iranian province.
"The right is reserved for the Islamic republic, as patron and heir to the territorial integrity of Iran, to want the return of a separated province to the Islamic homeland," said Hossein Shariatmadari, the director of the hardline Kayhan newspaper.
"The Bahrainis essentially consider themselves to be Iranians and according to some reports they are eager to return to Iran," he added without specifying on which reports.