(CNN) -- Foreign troops arrived Monday in the strategically and financially important Persian Gulf kingdom of Bahrain after a month of citizen protests, the Bahraini government said.
Saudi Arabia, Bahrain's giant neighbor to the west, appears to have provided at least some of the troops, who arrived under the banner of the Gulf Cooperation Council.
In a statement, the government described the troops as "coalition forces" but did not say what countries were represented. Their mission was equally vague: "The GCC Peninsula Shield coalition forces arrived in the Kingdom of Bahrain today following recent events, to help protect the safety of citizens, residents and critical infrastructure," it said.
The Saudi state news agency said its government had responded to Bahrain's request for help in view of the importance of security there.
According to the state news agency of the United Arab Emirates, southwest of Bahrain, it too "decided to send a security force to keep the peace in the Kingdom of Bahrain" at that country's request.
Anwar Mohammed Gargash, the UAE minister for foreign affairs, described the move as part of his country's responsibility within the Gulf Cooperation Council to bring "security and stability to the region."
In Paris, after a meeting with U.S. Secretary of State Hillary Clinton, the foreign minister of the United Arab Emirates, Abdullah bin Zayed al Nahyan, told reporters that the Bahrain government asked the council to "look at ways to help them to defuse the tension in Bahrain." The U.A.E. is a member of the Gulf Coalition Council, and the minister said his country had sent approximately 500 police officers to Bahrain to assist.
It was not clear how many foreign security troops had entered Bahrain in total. Various parts of the Bahraini government referred CNN questions to other government offices on Monday.
A witness said dozens of armored vehicles and buses full of soldiers crossed Monday afternoon from Saudi Arabia into Bahrain afternoon via the causeway linking the two countries.
The Gulf Cooperation Council comprises six Gulf states -- Bahrain, Saudi Arabia, Kuwait, the United Arab Emirates, Oman and Qatar -- and encourages cooperation among members in a number of areas, including the economy and security.
The Obama administration and U.N. Secretary-General Ban Ki-moon voiced concerns about the arrival of foreign troops in Bahrain.
"We urge restraint, we urge non-violence in response to non-violent protesters, the respect for the universal rights of people in the region to gather peacefully, to voice their opinions, to have their grievances heard by their governments, and to have greater participation in the political process," said Obama administration spokesman Jay Carney.
A spokesperson for Ban echoed that sentiment.
"The Secretary-General is troubled by the growing violence in the Kingdom of Bahrain that has left many people injured over the past few days," said spokesperson Anne Siddall in a prepared statement. "He notes with concern that troops from the Kingdom of Saudi Arabia and the United Arab Emirates under the auspices of the Gulf Cooperation Council have reportedly entered the territory of Bahrain today."
"The Secretary-General strongly believes that peaceful means should be adopted to ensure national unity and stability. He appeals to all concerned to exercise maximum restraint and to do everything possible to prevent the use of force and further violence," Siddall's statement read.
The movement of forces came on the same day that protesters seized control of a key part of the capital city of Manama, a Human Rights Watch official said.
About 100 demonstrators blocked access to the Bahrain Financial Harbour with barricades such as trash cans and cinderblocks, in effect shutting down the commercial district, Faraz Sanei said.
There was no police presence, he added.
"What we are witnessing in Manama is no peaceful protest," Bahrain's Foreign Minister Khalid al-Khalifa said. "It's wanton, gangster-style takeover of people's lives," he said on Twitter.
A pro-government group of lawmakers was urging the king of Bahrain to impose martial law for three months in the wake of the protests.
The arrival of the troops followed a day of clashes between protesters and security forces that resulted in the hospitalization of more than 1,000 people, human rights activists said.
The protests were the latest in a series that has swept across the Arab world this year, toppling the leaders of Tunisia and Egypt, but it was not clear that any other country had taken the step of calling in foreign troops for help.
"Temporarily, it should calm the situation," said Simon Henderson, director of the Gulf and energy policy program at the Washington Institute for Near East Policy. He noted that the Obama administration has been urging political dialogue but said Monday's move was not what the U.S. president meant.
"Sending in Saudi forces is hardly encouraging political dialogue," Henderson said. "The great danger is, it will actually worsen the situation by encouraging Iran to get involved. Not militarily, probably, but certainly diplomatically and rhetorically."
The underlying concern is that Iran, an overwhelmingly Shiite state, could seize the opportunity to meddle in Bahrain's internal affairs. Bahrain has a Shiite majority population, but its rulers are Sunni.
Saudi Arabia's eastern province is home not only to many of the country's rich oil fields but to its largest concentration of minority Shiite as well. In recent weeks, Shiite demonstrators there have protested the Saudi government, whose leaders are overwhelmingly Sunni.
The Saudi government would presumably be concerned that any uprising by Shiite Muslims in Bahrain could inspire the Shiite population in nearby Saudi Arabia to follow suit.
Protests on Sunday appeared to have been among the most violent since last month, when police tried to clear the capital's Pearl Roundabout, leaving seven people dead, according to demonstrators.
Most of Sunday's injured suffered gas-related injuries, including burns and breathing problems, according to Nabeel Rajab, president of the Bahrain Center for Human Rights. Doctors and nurses were among the injured. At least five people were in critical condition, and at least two people lost their eyes because of bullet injuries, he said.
The government denied accusations Sunday that unjustified force was used against protesters at the harbor, along a key highway and at Bahrain University.
Britain's Foreign Office warned Sunday against all travel to the Gulf kingdom until further notice, saying, "The risk of further outbreaks of violence has increased."
The nation's Independent Bloc of lawmakers called on Bahrain security forces to intervene to protect national security and stability, the Bahrain News Agency reported Sunday. The bloc is composed of the 22 pro-government members of the lower house of the legislature.
"Extremist movements are resorting to escalation and sectarian mobilization, which led to an unprecedented disruption of security and hostile sectarian polarization at health and educational institutions," the group said in a statement.
The members of parliament asked King Hamad to enforce a curfew and deploy security forces across the country.
During protests in the tiny island nation, moderates have been demanding a constitutional monarchy, and hardliners have called for the abolition of the royal family altogether.
CNN's Caroline Faraj, Mohammed Jamjoom and Jenifer Fenton contributed to this report.