Cairo - Saudi Arabian troops entered Bahrain on Monday as part of a military force from Gulf states called in to deal with a month of political unrest in the island kingdom.

Bahrain's government called in forces from its Sunni neighbors to put down unrest after protesters overwhelmed police and blocked roads in a resurgence of mass protests seen last month.

Nabeel al-Hamer, a former information minister and adviser to the royal court, said on his Twitter feed these troops were already on the island, a key U.S. ally and headquarters of the U.S. Navy's Fifth Fleet. Saudi officials declined comment.

Reporters saw no immediate movement of Saudi forces across the 16-mile causeway between the two countries.

Bahraini opposition groups including the largest Shiite Muslim party Wefaq said any intervention by Gulf Arab forces on the island was a declaration of war and an occupation.

"Forces from the Gulf Cooperation Council have arrived in Bahrain to maintain order and security," al-Hamer said.

Gulf Daily News, a newspaper close to Bahrain's powerful prime minister, reported on Monday that forces from the GCC -- a six-member regional bloc comprising Saudi Arabia, the United Arab Emirates, Kuwait, Qatar and Oman in addition to Bahrain -- would protect strategic facilities.

The GCC forces would help maintain law and order, it said in a front page report.

"Their mission will be limited to protecting vital facilities, such as oil, electricity and water installations, and financial and banking facilities."

Saudi-owned news network Al Arabiya also quoted an official source saying Bahrain would be supported by forces from the GCC, a military, economic and political alliance.

The reports come after Bahraini police clashes on Sunday with mostly Shiite demonstrators in one of the most violent confrontations since troops killed seven protesters last month.

After trying to push back demonstrators for several hours, police backed away and youths built barricades across the highway to the main financial district of the Gulf banking hub.

Those barricades were still up on Monday morning, with protesters checking cars at the entrance to the Pearl roundabout, the focal point of weeks of protests. On the other side of the same highway, police set up a roadblock preventing any cars moving from the airport towards the financial harbor.

Police were out in force in some areas but there was no evidence of soldiers, Bahraini or otherwise in Manama.

The GCC members are Bahrain, Saudi Arabia, Kuwait, Qatar, Oman and the United Arab Emirates.