The main fear in the region is the conflict could spread to Arab states, particularly those with sharp sectarian divisions, such as Iraq and Lebanon.
Saudi Arabia, the world's leading oil producer and the birthplace of Islam in the seventh century, has a sizeable Shiite minority, which predominates in the kingdom's Eastern province.
That's Saudi Arabia's main oil-producing zone, so it's highly sensitive and Riyadh has long feared that Shiite-dominated Iran seeks to use its co-religionists as a fifth column to sabotage its all-important oil industry.
Turkey, a Sunni-dominated state that under its Islamic government is striving to restore its Ottoman-era power and become the paramount state in the region.
Turkey, neighbor to both Iran and Syria, is increasingly being drawn into the Syrian conflict against the Damascus regime, its former ally.
"It's becoming increasingly clear that the Syrian uprising transcends the strategic interests of Iran and Turkey, as it has become the battleground between the Sunni and Shiite communities throughout the Middle East," observed Alon Ben-Meir, professor of international relations and Middle East studies at New York University's Center for Global Affairs.
"The new political order that will eventually emerge in Syria will determine not only the ultimate success or failure of Iran's aspirations to become the region's hegemon but whether or not the Sunni Arab world will maintain its dominance.
"Hence, the conflict will be long, costly and bloody, reflecting the troubled history between the two sides that has extended over a millennium," Ben-Meir wrote in a Huffington Post analysis published April 13.
Sectarian conflict is simmering in Iraq in the wake of the completion of the U.S. military withdrawal Dec. 18, 2011, largely involving wide-ranging bombings by al-Qaida's Sunni fighters against the country's Shiite majority.
Sunni radicals from Iraq are reported to be among al-Qaida fighters moving into Syria, Iraq's northwestern neighbor, to join their co-religionists battling to bring down the Damascus regime dominated by the minority Alawite sect, an offshoot of Shiite Islam backed by Tehran.
There's growing alarm among Iraq's minority Sunnis, who dominated power under Saddam Hussein, that Prime Minister Nouri al-Maliki, a Shiite with links to Tehran, is seeking to impose a new dictatorship after decades of brutal repression of Shiites by Saddam's Baathists.
The Saudis and other Sunni states, such as the Persian Gulf monarchies, Egypt and Jordan, fear Iran will dominate Iraq, with its Shiite majority, now that the Americans have departed and are widely considered to be funding the Sunni militants.
Sectarian divisions in Lebanon, where the Iran- and Syria-backed Shiite Hezbollah is locked in an increasingly bitter political struggle with Western-supporter Sunnis and their allies, have been dangerously sharpened since the Syrian uprising began March 15, 2011.
Indeed, these divisions have never been far from the surface since the end of Lebanon's 1975-90 civil war.
Those divisions were infinitely widened in 2011 after a United Nations-mandated tribunal charged Hezbollah activists with the Feb. 15, 2005, Beirut assassination of former Prime Minister Rafik Hariri, Lebanon's foremost statesman and the main leader of its Sunnis.
Hezbollah denies the charges and has warned it will "cut off the hands" of anyone who seeks to apprehend four of its members, two of them senior figures, indicted for the bombing that killed Hariri and 22 others.
Hezbollah supports the Syrian regime of President Bashar al-Assad, while most Lebanese Sunnis oppose it.
Under Hariri, they campaigned for Syria to withdraw the quasi-occupation forces Damascus had deployed in Lebanon since 1976. After Hariri was slain, international pressure finally forced the Syrians to pull out, although they remain a powerful influence in Lebanon.
Shiites and Sunnis have been slaughtering each other in Pakistan for years and the Pakistani Sunni militants of Lashkar-e Jhavangi recently claimed responsibility for bomb attacks against Shiites in the Afghan cities of Kabul, Mazar-e-Sharif and Kandahar.
The Pakistani group, known as LeJ, has close ties to al-Qaida.
In January, Turkish Foreign Minister Ahmet Davutoglu warned against a regional sectarian conflict.
"There are some who are willing to start a regional Cold War," he declared. "We're determined to prevent that. Sectarian tensions would be suicide for the whole region."