Television footage showed convoys of unmarked, desert brown armoured vehicles crossing from Saudi's Eastern Province into Bahrain, the home of the US Fifth Fleet.

The Saudi government said it had responded to a call for help from its neighbour as Saudi led forces from the Gulf countries' joint Peninsula Shield Force crossed the causeway into Bahrain.

"The council of ministers has confirmed that it has answered a request by Bahrain for support," the Saudi government said in a statement carried by the SPA state news agency.

Under an agreement of the six country Gulf Cooperation Council (GCC), "any harm done to the security of a member state is considered a harm done to the security of all members," it said.

The United Arab Emirates also said it had sent some 500 police to help "defuse tension," according to Foreign Minister Sheikh Abdullah bin Zayed al-Nahayan.

It is not clear the other GCC members Kuwait, Oman and Qatar were also participating on this call.