The Free Syrian Army, one of the leading groups within the overwhelmingly Sunni Muslim opposition, threatened Tuesday to strike at the Shiite Hezbollah in Lebanon after the Iranian-backed movement sought to extend its control of Syrian territory along the border.
The Hezbollah offensive, which began last week, appears intended to protect vital supply routes to Syrian forces loyal to the regime from Hezbollah's heartland in the Bekaa Valley in northeastern Lebanon.
Hezbollah also needs to maintain its own supply route from Syria along which it has been reportedly receiving advanced weapons, such as surface-to-surface missiles capable of hitting anywhere in Israel, and Russian SA-17 anti-aircraft weapons, from President Bashar Assad's regime and Iran.
Israel views this flow of weapons with considerable alarm because, among other things, they will allow Hezbollah for the first time to challenge its air supremacy.
This is the most likely issue to draw the Israelis into Syria's war and a Jan. 30 airstrike, supposedly against a Hezbollah arms convoy, indicated how jumpy the Israelis are getting about Iran's support for Assad, its key Arab ally.
The recent assassination in Syria of a senior Iranian general involved in this operation demonstrated how the Syrian conflict may be expanding.
There is still confusion over when and where Gen. Hassan Shateri of the elite Al-Quds Force, the covert action arms of Iran's Revolutionary Guards, was killed.
The FSA claims he was killed in the Jan. 30 airstrike and may have been the target of that raid, about which Israel has maintained a stony silence. Tehran said he was ambushed Feb. 11 by rebels.
So Hezbollah's push on the border may have wider implications. One report said Hezbollah had deployed 1,000 fighters, partly to relieve Syrian troops needed to block rebel advances in the north.
Hezbollah has sought to expand the control it has over a cluster of some 20 Shiite villages on the Syrian side of the porous border by seizing nearby Sunni villages where the FSA's deployed.
These have long been involved in smuggling and are an important route for arms, fighters and other supplies to regime forces around the town of Qusayr, where many Lebanese Shiites reside and where there has been heavy fighting.
West of Qusayr lies the strategic city of Homs, a major battleground.
There have been casualties on both sides in the Qusayr clashes, although figures are hard to come by.
FSA spokesman Louay al-Miqdad termed Hezbollah's push as "an unprecedented invasion ... the first of its kind in terms of organization, planning and coordination with the Syrian regime's air force ...
"If Hezbollah does not stop its land invasion, which is accompanied by covering fire, we are allowed to retaliate ... If they do not stop, we will retaliate."
That would mark a sharp escalation in the Syrian conflict and would likely ignite broader Sunni-Shiite violence in Lebanon that many have feared has been steadily coming to the boil in recent weeks as the Syrian war sharpened long-simmering sectarian animosities.
Sunni sources in Beirut have been talking darkly of "something big" expected in the coming days, hinting at action by Sunni jihadists linked to those fighting in Syria, most notably the Jabhat al-Nusrah, or the Al-Nusrah Front.
Hezbollah is widely believed to have substantial forces in Syria fighting alongside Assad's military and his murderous paramilitary force, the Shabiha, or Ghosts.
Some 50,000 of Assad's Alawites have recently been recruited to bolster regime forces as the rebels gain ground, spearheaded by the FSA and Jabhat al-Nusrah, the most ruthless of the anti-regime groups.
The Front reportedly demonstrated its capabilities recently, claiming it decimated a Syrian army convoy in an elaborate ambush using 50 roadside bombs detonated simultaneously in the eastern Ghotah district of Damascus.
A Front video of the massive ambush obtained by the SITE intelligence group indicated the spectacular attack took place Dec. 6.
The irony is that elaborate roadside ambushes like this were pioneered by Hezbollah in the 1990s in its campaign in south Lebanon against Israeli occupation forces who eventually withdrew in May 2000.