The 1,000 Saudi troops now in Bahrain are there at the request of the Bahrain government, but there are fears that the presence of foreign troops could be a major provocation for anti-government protesters who have already clashed violently with security forces.
Seven protesters were shot dead at a rally against the ruling Sunni monarchy.
In the U.S., Pentagon officials said they had been given no advance warning that Saudi Arabia would be sending the troops to Bahrain, despite the fact that Washington counts both countries as its close allies in the region.
Bahrain’s majority Shia population has been calling for reforms since popular unrest toppled the governments of Egypt and Tunisia earlier this year.