The foray into Syria's civil war by Lebanon's Hezbollah has fuelled a Sunni-Shiite polarisation that threatens to feed extremism on both sides and export the conflict to the wider region, analysts warn.
The Iran-backed Shiite movement has openly said it is fighting alongside President Bashar Al-Assad's forces, while Shiite Iraqi fighters are also reported to be in Syria, supporting the regime against the mostly-Sunni rebels.
These interventions have prompted calls for a united Sunni stance against the Shiite groups involved, particularly Hezbollah.
Sunni-dominated Saudi Arabia's top cleric Abdulaziz al-Shaikh has urged governments to punish the "repulsive sectarian group" while Qatar-based Sunni cleric Yusuf al-Qaradawi called on Sunnis to join the rebels.
George Sabra, interim head of the opposition Syrian National Coalition, charged Hezbollah, along with majority-Shiite Iraq and Iran, of pushing the situation towards a "sectarian conflict".
"What we are fearing now is that the whole region could drown in a sectarian-fuelled conflict which in effect is a series of civil wars including Lebanon, Iraq, and of course Syria itself," says Salman Shaikh, director of the Brookings Doha Centre.
Fighters from Shiite Hezbollah openly spearheaded a 17-day assault on the Syrian town of Qusayr near the Lebanese border which culminated on Wednesday with its recapture from the rebels by pro-government forces.
The battle for Qusayr further stoked the already-simmering sectarian tension across the region, the analysts say.
Assad's regime is dominated by members of the Alawite minority, an offshoot of Shiite Islam, while Sunnis make up the majority in Syria and the Muslim world.
Hezbollah's "association with the conflict on sectarian lines is creating tensions in Lebanon and in the wider Arab world," says Shaikh.
Iraqi Prime Minister Nuri al-Maliki, whose Shiite government is facing a wide Sunni-led opposition, warned Sunday of "a storm passing through the region. It is a brutal sectarian storm."
Funerals were held in Iraq last month for men killed in Syria fighting alongside Assad's forces.
Emirati political science professor Abdulkhaleq Abdulla says that "the sectarian line-up has recently reached worrying levels."
Although a historical conflict, the Sunni-Shiite divide is "now different... because it has become based more on a political background than a religious one," says Abdulla.
Saudi Arabia, home to Islam's holiest sites, is seen the regional power protecting Sunnis while Iran has become a reference for all Shiites, he says.
Lebanese columnist Hazem Sagheye sees that the Syrian crisis "has morphed into a cross-border Sunni-Shiite line-up."
He argues that Damascus can stir trouble in surrounding countries through "holding cards" -- groups loyal to Assad's regime.
Lebanon officially adopted a position of neutrality towards Syria's conflict but its people are sharply divided with Shiites mostly backing Assad while most of the Sunnis support the rebellion.
Fighters from both sects have joined the battle on opposite sides.
This division is clearly reflected in frequent deadly clashes in Lebanon's second-largest city, Tripoli, between Sunni and Alawite gunmen. The army warned on Friday of a plot to embroil Lebanon in the 26-month Syrian conflict.
And while Sunni figures roundly condemned Hezbollah's involvement, news of the fall of Qusayr sparked celebrations in Lebanon's Shiite districts.
Reaction also came from the Gulf kingdom of Bahrain -- the scene of unrest between a Shiite opposition and the Sunni monarchy -- with the Shiite Unitary National Democratic Assembly issuing a congratulatory statement.
"What is scary is that the rise in sectarianism could once more ignite Al-Qaeda and extremism, posing a danger to the region," says prominent Saudi columnist Tariq Alhomayed.
And a decades-long standoff between Iran and Saudi Arabia appears now to be playing out by proxy in Syria, Yemen, Bahrain and Lebanon, where the rivals are supporting opposing political strands.
The Syrian opposition recently charged that the battle in their country has even attracted Shiite Zaidi rebels from Yemen to join the fight alongside Assad's troops. Zaidis have denied the claims.
But while Shiite armed groups fighting in Syria are openly backed by Iran, Sunni Islamist fighters trickle into Syria as individuals and mostly against the wish of their own states.
Saudi Arabia has repeatedly warned its citizens against taking part in the conflict.
Damascus' brutal repression of protests, in addition to Iran's full support to Assad and Hezbollah's intervention, have "emphasised the Sunni character of the other side," says Sagheye.
With the stirring of sectarian sentiment across the region and the mushrooming of armed extremist Sunni and Shiite groups, the region risks "a collapse of the concept of the state, with every group having its own media outlets and militias," warns Alhomayed.
"We are moving closer towards chaos in the Arab world."