MANAMA (Reuters) - When Saudi Arabian troops rolled into Bahrain to help quell Shi'ite Muslim protests, the world's top oil-exporting region inched closer to a sectarian stand-off that could involve non-Arab Shi'ite power Iran.
Gripped by its worst unrest since the 1990s after protesters took to the streets last month, Bahrain said on Monday it had asked the Gulf Cooperation Council, a six-member Gulf Arab bloc, for support in line with a regional defense pact.
The intervention of Gulf Arab troops in Bahrain is highly sensitive on the island, where the Shi'ite majority complains of discrimination by the royal al-Khalifa family, who are Sunnis.
Gulf Arab ruling families are Sunni and intervention might encourage a response from Iran, which already supports Shi'ite groups in Iraq and Lebanon, countries with large Shi'ite populations and no strangers to persistent sectarian strife.
Iran reacted swiftly, urging Bahrain not to allow what it called foreign interference in dealing with appeals for reform.
"Using other countries' military forces to oppress these demands is not the solution," Foreign Ministry official Hossein Amir Abdollahian told the semi-official Fars news agency.
Accusations already abound of Iranian backing for Shi'ite activists in U.S.-allied Bahrain -- charges they deny.
"Although this is unlikely at this stage, what may be perceived by some Bahrainis as foreign intervention may lead to calls by some for non-GCC intervention, such as from Iran, to protect the Shiites in the kingdom," said Ghanem Nuseibeh, partner at consultancy Cornerstone Global.
"Should the confrontation between the rioters and the non-Bahraini forces lead to substantial casualties, the likelihood of this happening will substantially increase."
Bahraini opposition groups, including the largest Shi'ite party Wefaq, have already said they consider the move an act of undeclared war against defenceless civilians.
Civilians in Bahrain are unarmed. Sectarian clashes that broke out around the country in recent days involved clubs, knives, fists and rock-throwing. They would need foreign assistance if they were to mount any resistance to Gulf troops.
Saudi intervention began after veiled warnings to Iran from the GCC Secretary-General Abdul Rahman Attiyah, who said the bloc would not accept foreign intervention in Bahrain.
Analysts saw the troop movement as a mark of concern in Saudi Arabia that concessions by Bahrain's monarchy could inspire the conservative Sunni kingdom's own Shi'ite minority.
Shi'ites make up about 15 percent of the population in Saudi Arabia, a key U.S. ally and the world's top exporter of oil.
Saudi protests have mostly taken place in the Eastern Province, where the oil industry is based and which is home to most of the Shi'ites in the Sunni kingdom. As the neighbours are joined by a bridge, the risks of contagion are lost on no one.
Political analysts said the Saudi military move suggested Bahrain's royal family had decided to reject repeated U.S. pleas, made most recently by visiting U.S. Defense Secretary Robert Gates on Saturday, for a more conciliatory approach.
"There has been a struggle in terms of policy advice between U.S. and Saudi Arabia," said Marina Ottaway, head of the Middle East Program at the Carnegie Endowment for International Peace.
"The U.S. has been trying to get the Bahraini government to respond by negotiation, by reform and by dialogue. The Saudis have been saying that they have to put the uprising down. They have decided to listen to the Saudis."
Bahrain's call for help is significant; GCC troops were last deployed under the aegis of the Peninsula Shield in Kuwait, during the 2003 invasion of Iraq, and in the 1990-1 Gulf War.
So far numbering 1,000 Saudi troops, the Gulf forces' mission is to help restore law and order after weeks of protests by mainly Shi'ite demonstrators against the government and to protect key facilities such as oil, electricity and water installations and financial and banking institutions.
The intervention comes a day after protesters overwhelmed Bahraini police in one of the most violent confrontations since troops killed seven protesters last month.
Riad Kahwaji, analyst at Institute for Near East & Gulf Military Analysis, said Bahrain could make use of special operations and infantry but that force may complicate matters.
"We have lessons learned from Tunisia and Egypt about the use of force. This is not the sort of conflict where use of force brings progress. This is the sort of conflict where you need to engage with openmindness via dialogue," he told Reuters.
In a sign talks may begin soon, Wefaq said the opposition had met the crown prince to discuss a mechanism for dialogue.
Sheikh Salman al-Khalifa offered assurances on Sunday talks would address key opposition demands including giving parliament more power and reforming government and electoral districts.
Even if talks are successful, however, the opposition is increasingly split and hardline groups may keep up protests.
Wefaq is calling for a new government and a constitutional monarchy that vests the judicial, executive and legislative authority with the people. A coalition of much smaller Shi'ite groups are calling for the overthrow of the monarchy -- demands that scare Sunnis who fear this would benefit Iran.
"Military intervention from GCC countries means the situation is increasingly untenable for the regime. The paradox the Bahraini regime faces is that it cannot contain the unrest while trying to kick off talks," consultancy Stratfor said.
"Wefaq... risks losing ground against hardliners if it appears too close to the regime while Shi'ite protesters are beaten by the police."
Additional reporting by Andrew Hammond in Dubai, Ulf Laessing in Riyadh, Peter Apps in London and Andrew Quinn in Washington; editing by Ralph Boulton