An authoritarian Arab ruler unleashes his security forces and irregular militia gunmen to crush peaceful pro-democracy protests, killing hundreds of people including women and children.

Does the West a) issue statements condemning the excessive use of force; b) seek U.N. sanctions and an International Criminal Court investigation; c) provide practical support for pro-democracy protesters, d) intervene militarily?

The answer, to many human rights campaigners, seems to vary unacceptably depending on the state concerned.

Western powers which took up arms against Muammar Gaddafi in Libya, citing the United Nations principle of the responsibility to protect civilians, have confined themselves so far to verbal outrage at the killing of some 350 people in Syria.

The balance of Western economic and security interests and humanitarian values is different in each case but the perceived double standard is causing anger in the Middle East and among Western publics.

"After Friday's carnage, it is no longer enough to condemn the violence," Joe Stork, deputy Middle East director at pressure group Human Rights Watch, said in a statement.

"Faced with the Syrian authorities' 'shoot to kill' strategy, the international community needs to impose sanctions on those ordering the shooting of protesters."

When the Gulf Arab kingdom of Bahrain called in Saudi troops last month to help quash a pro-democracy movement led mostly by the Shi'ite Muslim majority, the United States and Europe uttered a few pro-forma words of disapproval, then fell silent.

"DIFFERENT SITUATIONS"

The killing in Bahrain was on a smaller scale than in Libya or Syria, and the ensuing arrests, dismissals and disappearances of opponents have drawn less media coverage.

More importantly, Bahrain is home to the U.S. Fifth Fleet, which keeps an eye on Shi'ite Iran across the Gulf and patrols the world's most vital oil-export sea lanes.

The ruling family in the Gulf island state is so close to former colonial power Britain that the crown prince was invited to this week's royal wedding in London until he declined the invitation to spare British embarrassment.

There are strategic, political and practical reasons behind divergent Western responses to events in Syria, Libya and Yemen, after the initially hesitant Western embrace of democratic change in Tunisia and Egypt.

"All of these situations are different," British Foreign Secretary William Hague said on BBC television when questioned about apparent inconsistency.

"So we mustn't think that just because we're doing certain things in Libya, that we would be able or willing to do those things in other countries of the Arab world."

Hague said that in Libya, there was a direct appeal for help from the opposition and the Arab League had asked the U.N. Security Council to pass a resolution and to take action for a no fly zone. Western governments say they prevented an imminent massacre that Gaddafi had threatened to unleash in Benghazi.

Gaddafi had lost control of more than a third of his country and his armed forces were brittle and poorly equipped.

By contrast, Syria has a well-trained army with Russian missiles and combat aircraft, and suspected chemical weapons, making any Western military intervention utterly implausible.

AVOID Destabilization

A key strategic consideration is that the West is keen to ensure that Arab uprisings, and the rulers' responses, do not destabilize the entire Middle East, threatening oil supplies to the industrialized world or triggering wider conflict.

Oil has already risen to nearly $125 a barrel from around $80 last year, partly due to a drop in Libyan supply but also because Saudi Arabia has cut back output, forcing prices up.

Riyadh's move is seen partly as driven by the need to fund huge hand-outs promised by King Abdullah to interest groups to pre-empt any possible unrest in his absolute monarchy.

It may also reflect tension between Saudi Arabia and Washington. Some diplomats say Saudi rulers were incensed by the way U.S. President Barack Obama dropped Egyptian President Hosni Mubarak, a veteran pro-American stalwart in the region.

Since the overthrow of Saddam Hussein in Iraq, the United States' priority in the region has been to prevent Iran acquiring a nuclear weapons capability which Western and Arab strategists say would be profoundly destabilizing.

Syria is Iran's closest ally and Western powers have been trying for the last two years to woo President Bashar al-Assad away from Tehran and encourage the British-trained eye doctor to reach a peace deal with Israel that could remove a major source of regional friction.

After years of unsuccessfully trying to corner Syria over the 2005 assassination of Lebanese Prime Minister Rafik Hariri, the United States sent an ambassador back to Damascus this year.

France appointed a special envoy last summer to facilitate back channel contacts between Syria and Israel, and Turkey, Spain and Germany have also been involved in passing messages, diplomats say.

If the West seeks a Security Council condemnation of Assad or a referral of the repression to the ICC, Russia, an historic ally of Libya, may well veto any resolution.

Western diplomatic action could push Syria more tightly into the arms of Iran and risk retaliation by Syrian-allied Hezbollah forces in Lebanon either against Israel or European troops policing a southern Lebanese buffer zone.

Western diplomats say they are also concerned at the risk of sectarian conflict in Syria, dominated for nearly five decades by an Alawite minority close to Shi'a Islam. Violence involving Sunni Arabs, Alawites, Kurds and Druze could embroil neighboring Turkey, Lebanon and Iraq.

So while Western governments are likely to step up rhetoric against Assad and explore the scope for targeted U.N. sanctions and an ICC referral, there is little they can do to affect the outcome of the popular uprising.

(Editing by Angus MacSwan)