MANAMA, Bahrain — Hundreds of troops from Saudi Arabia and police officers from the nearby United Arab Emirates have entered Bahrain at the request of the ruling family, a move that further polarized the tiny island nation and marks the first time Arab nations have intervened in another country's affairs amid sweeping unrest in the region.

MANAMA, Bahrain — Hundreds of troops from Saudi Arabia and police officers from the nearby United Arab Emirates have entered Bahrain at the request of the ruling family, a move that further polarized the tiny island nation and marks the first time Arab nations have intervened in another country's affairs amid sweeping unrest in the region.

Bahrain television showed a line of armored vehicles Monday carrying Saudi soldiers crossing the 16-mile King Fahd Causeway that links the two countries. The surprise deployment came after several days of worsening violence that had paralyzed the country and threatened to bring down the monarchy.

But if the intent of Bahrain's ruling Khalifa family was to shore up its precarious position, it seemed at least as probable that bringing in Saudi troops would worsen the crisis by raising the chance of violence and uniting the often-fractious opposition behind a single issue: a refusal to yield to outside military pressure.

After learning of the Saudi troops' arrival, demonstrators began expanding the barricades and checkpoints they have set up to keep authorities out of the tent city that has arisen at the Pearl roundabout. Demonstrators have occupied the traffic circle since February to protest what they say is systematic discrimination against majority Shiite Muslims by the country's Sunni rulers. "If they send them, they will kill us," protester Abdullah Ali said of Saudi troops. "We are ready to be killed. Everyone's ready here."

In a statement, the White House urged Saudi Arabia and any other countries that might dispatch troops "to show restraint and respect the rights of the people of Bahrain, and to act in a way that supports dialogue instead of undermining it," said Tommy Vietor, a spokesman for the National Security Council.

Still, many Arabs will believe that the Saudis entered Bahrain with tacit U.S. approval, especially as the arrival comes a few days after U.S. Defense Secretary Robert Gates visited Bahrain, said Toby C. Jones, professor of modern Middle Eastern history at Rutgers University.

The Pentagon said Gates received no notification during his visit of any planned intervention by the Saudis.

The United States has found itself in the awkward position of having to balance seemingly contradictory positions as protests have roiled autocratic North African and Middle Eastern countries. In the case of Egypt and Libya, the White House said the strongmen in power there had lost the legitimacy to lead. For staunch ally Bahrain, however, the U.S. has continued to urge the regime to negotiate with protesters.

But Bahrain's royal family may have felt it was running out of options. Since the protests began, it has faced pressure from Saudi Arabia to not yield to the protesters, while the Obama administration has called on it to address their demands and criticized the use of force early in the crisis.

Saudi Arabia has long feared that unrest in Bahrain would spur protests among its own Shiite population, who are mainly concentrated in the country's oil-rich Eastern province adjacent to Bahrain.

Since the unrest began in Bahrain, U.S. officials have made it clear they want to see stability restored quickly in the country, which is home to the headquarters of the U.S. Navy's 5th Fleet, but not by use of force.

Bahrain's crown prince, Sheik Salman bin Hamad Khalifa, has made offers of talks to the protesters, but the opposition has remained deeply divided and so far unwilling to take up the government's offer for dialogue to resolve grievances.