Bahrain has asked neighbouring Gulf states to intervene after violent clashes broke out between police and protestors on Sunday.
An advisor to the royal court said forces had arrived in the island kingdom in a bid to restore order to the streets.
"Forces from the Gulf Cooperation Council have arrived in Bahrain to maintain order and security," Nabeel al-Hamer, a former information minister and adviser to the royal court, was quoted as saying on his Twitter feed by Reuters.
State-linked newspaper Gulf Daily News said security forces would be used to protect key facilities, after violent confrontations overwhelmed local police yesterday.
"GCC forces will arrive in Bahrain today to take part in maintaining law and order," the paper said.
"Their mission will be limited to protecting vital facilities, such as oil, electricity and water installations, and financial and banking facilities."
The news prompted the British Foreign Office to issue an alert warning against all travel to Bahrain, and urging British nationals in the Gulf kingdom to remain at home.
In a statement, it said there were fresh reports that Saudi soldiers were about to enter the country to intervene in the revolt.
“The risk of further outbreaks of violence has increased,” the statement said.
Thousands of demonstrators on Sunday cut off Bahrain's financial centre and drove back police trying to eject them from the capital's central square, in one of the most violent confrontations since the military killed seven protesters on February 17.
Police fired tear gas and water cannons at the crowds who erected barricades across the highway after overwhelming riot police near Manama’s Pearl roundabout.
Bahrain, which is connected to Saudia Arabia via a sea causeway, has been gripped by its worst unrest since the 1990s after protesters took to the streets last month, inspired by uprisings that toppled the leaders in Egypt and Tunisia.
The kingdom has seen weeks of rallies by its disgruntled Shi'ite majority, which says it is discriminated against by the Sunni royal family.
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