WASHINGTON - The State Department on Monday urged U.S. citizens to avoid traveling to Bahrain due to "the potential for ongoing political and civil unrest."
The department also authorized the voluntary departure of eligible family members of U.S. Embassy staff, and advised other Americans currently in Bahrain to consider leaving.
Bahrain has been swept up in the wave of protests that have arisen in the Arab world since December. On Monday, a military force from Saudi Arabia and other Gulf nations moved into Bahrain to shore up its Sunni Muslim rulers in the face of escalating Shiite-led protests.
"While demonstrations have not been directed toward Westerners, U.S. citizens are urged to remain alert to local security developments and to be vigilant regarding their personal security," the State Department said in its travel warning, upgraded from an earlier travel alert.
The U.S. also urged Americans "to avoid all demonstrations, as even peaceful ones can quickly become unruly and a foreigner could become a target of harassment or worse."
The Saudi-led force crossed into Bahrain to prop up the monarchy Monday in the first cross-border military operation to quell unrest since the Arab world's rebellions began in December.
Opposition groups immediately denounced Monday's intervention as an occupation that pushed Bahrain dangerously close to a state of "undeclared war."
Bahrain's majority Shiite Muslims see an opportunity to rid themselves of two centuries of rule by a Sunni monarchy. But Gulf Sunni leaders worry that any cracks in Bahrain's ruling system could threaten their own foundations. Protests are already flaring in Oman, Kuwait and even tightly ruled Saudi Arabia.
Gulf leaders are also concerned that political gains by Bahrain's Shiites might give Shiite Iran a stepping stone to its arch-rival Saudi Arabia, connected to Bahrain by a wide causeway.
Instead, the Saudis and other members of the Gulf Cooperation Council sent forces the other way, deploying about 1,000 troops by land and air and cementing the entire six-nation alliance to the fate of Bahrain's rulers, who are key U.S. allies as hosts of the American Navy's 5th Fleet.
Shortly after word of the foreign military reinforcements began spreading through Bahrain, protesters blocked roads in the capital Manama. Thousands of others swarmed into Pearl Square, the symbolic center of the monthlong revolt.
"No to occupation!" demonstrators cried in the square.
The worries about Iran have persisted, even though there are no apparent links between Tehran and Bahrain's opposition.
"The Gulf leaders have tried to legitimize this. They portray it not as intervention in an internal Bahrain dispute, but rather as an action against an external threat," said Sami Alfaraj, director of the Kuwait Center for Strategic Studies.
In Tehran, authorities had no comment on the Gulf force moving into Bahrain. Iranian Foreign Minister Ali Akbar Salehi called on Bahrain to avoid using "violence and force," according to the semiofficial Fars News Agency.
In Washington, White House spokesman Jay Carney pointedly did not call on the Saudi-led force to withdraw.
"We are calling on the countries in the region to show restraint and pointing to the fact that the dialogue that can bring about political reform is essential for the stability of the countries in the region and their continued economic prosperity," Carney said.
It's not clear when Bahrain sought outside help. The rulers have faced a month of nonstop unrest that has left seven people dead and the country drifting toward open sectarian conflict.
There have been scenes of defiance and disobedience so unsettling that pro-government parliament members appealed to Bahrain's king to impose martial law. On Sunday, protesters paralyzed Bahrain's finance district with roadblocks and then stood their ground and in some cases pressed forward against riot police firing tear gas in Pearl Square.
A statement on the state-run Bahrain News Agency said troops from the GCC's Peninsula Shield Force have been deployed "in line with the principle of common destiny bonding." The bloc is made up of Saudi Arabia, Oman, Kuwait, Bahrain, Qatar and the United Arab Emirates.
The reason for the mission was "the common responsibility of the GCC countries in maintaining security and stability," the statement said.
The Peninsula Shield Force was created in the 1980s. Military units under a GCC command have been sent to Kuwait, including during the 1991 U.S.-led campaign to oust Saddam Hussein's force and in 2003 before the invasion of Iraq. The current action marks a significant shift to help a government quell internal unrest.
Jane Kinninmont, a senior research fellow and Bahrain expert at the London-based think tank Chatham House, said Monday's operation "changes the role of the GCC," which has always had collective defense pacts.
"The idea of gathering together to protect a government against its own people seems to be quite another thing," Kinninmont said.
The United Arab Emirates and Saudi Arabia have announced their forces were sent to Bahrain, but it was unclear about the contributions from the other states.
By midday Monday, dozens of Saudi military vehicles were crossing over the causeway connecting the countries. The Gulf Daily News, which is close to Bahrain's rulers, said the Gulf forces would protect sites such as electricity stations and oil facilities.
Opposition leaders were outraged and in an ominous sign for Bahrain's rulers talked about resistance rather than possible dialogue.
"We consider that any military force or military equipment crossing the boundaries of Bahrain from air, sea or land an occupation and a conspiracy against the people of Bahrain ... and threatens them with an undeclared war by armed troops," said a statement from a coalition of seven main opposition groups.
Bahrain's leaders have expressed increasing frustration that opposition factions have not accepted offers to open talks.
In a series of Twitter messages, Bahrain's prime minister, Sheik Khalifa bin Salman Al Khalifa, lashed out at the protesters.
"What we are witnessing in Manama is no peaceful protest," he wrote. "It's wanton, gangster-style takeover of people's lives."
Shiites account for 70 percent of the population, and the main opposition groups have called for the Sunni rulers to give up most of their powers to the elected parliament. But as violence has deepened, many protesters now say they want to topple the entire royal family.
U.S. Defense Secretary Robert Gates visited Bahrain on Saturday and urged leaders to quickly move on reforms. On the same day, Bahrain's Interior Ministry said the sectarian strife was threatening the "social fabric" of the nation.
Last week, the GCC promised $10 billion in aid to both Bahrain and Oman, which also had faced protests calling for more jobs and a greater public voice in political affairs.