Bahrain's king said a foreign plot against his Sunni-led island state had been foiled, and the head of the Gulf Cooperation Council said interference by Shi'ite Iran in the Gulf Arab states would not be tolerated.

Confrontation between Sunnis and Shi'ites has stirred international tension in the oil-exporting region, gripped by its worst unrest in years.

"An external plot has been fomented for 20 to 30 years until the ground was ripe for subversive designs... I here announce the failure of the plot," King Hamad bin Isa Al Khalifa was quoted as telling troops in a report by state news agency BNA.

Had the plot succeeded, he said, it could have spilled into neighboring states.

The king thanked troops brought in from fellow Sunni-ruled neighbors to help quell weeks of unrest following protests by mainly Shi'ite Bahrainis calling for political reform.

He did not say who was behind the plot. The comments came after a day of tit-for-tat diplomatic expulsions between Bahrain and Shi'ite power Iran.

Gulf Cooperation Council (GCC) Secretary-General Abdulrahman al-Attiyah told reporters: "We reject any intervention in our internal affairs and among these countries (intervening) is Iran," after he was asked about troops from Saudi Arabia and the United Arab Emirates being sent to help the Bahrain government.

Saudi-owned Al Arabiya television channel quoted Bahrain authorities as saying intelligence communications systems had been sent from Iran to the Bahraini opposition.