Bulgaria's allegation that Hezbollah was behind a deadly bus bombing last summer has intensified international pressure on the Iran-backed group as it wrestles with the prospect its key ally, Syrian President Bashar Assad, may be toppled.

Hezbollah has indirectly denied the Iranian-backed Shiite movement was involved in the July 18, 2012, bombing at the Black Sea resort of Burgas in which five Israelis were killed.

But more damaging in the region's highly charged political climate these days, as the Syrian bloodbath approaches its third year and steadily spills into Lebanon, is the allegation that two plotters in the bombing are hiding in Lebanon.

If that's true, they're presumably under Hezbollah protection, as are four members, one a senior figure, indicted by a U.N.-mandated special tribunal for the Feb. 14, 2005, assassination of Rafik Hariri, a former Lebanese prime minister and the country's leading statesman.

The Bulgarians say one Burgas suspect has a bona fide Australian passport, the other a Canadian.

That brings to mind a special unit that Hezbollah recruited in the 1990s that comprised light-skinned men who'd lived in the West and could move without suspicion in Western societies.

Their mission was primarily to infiltrate Israel. At the time, Hezbollah wasn't conducting foreign operations, although it seems to be doing that now in conjunction with Iran's Al Quds Force, an elite covert unit of the Islamic Revolutionary Guards Corps.

The Israelis and Americans say the primary mission of the Hezbollah-Al Quds operation is to retaliate for the assassination of scientists engaged in Tehran's nuclear and ballistic missile programs. These killings over the last four years have been blamed on Israeli and U.S. intelligence services.

But so far, the Iranian-Hezbollah plots have largely been thwarted. Two dozen or so Iranians and Hezbollah members have been captured on bungled or intercepted missions in Azerbaijan, India, Thailand, Cyprus and elsewhere over the last couple of years.

Hezbollah has also sworn to avenge its iconic military chief, Imad Mughniyeh, assassinated in a car bomb in Damascus Feb. 12, 2008. Israel was blamed for that. It's never acknowledged responsibility but the operation had the signature of Israel's intelligence services written all over it.

The assassination was a carbon copy of the killing of several top militants in the West Bank during the 2000-03 Palestinian uprising.

Since Mughniyeh's death, Hezbollah's special operations units have been run by his brother-in-law, Mustafa Badreddine, and a former Mughniyeh lieutenant, Talal Hamiyeh.

These days, Hezbollah's main operational focus is helping Assad to save his minority regime, which is dominated by the Alawite sect, an esoteric offshoot of Shiite Islam.

Hezbollah's reported to have sent a sizeable force into Syria to bolster Assad's troops fighting rebels largely from Syria's long-suppressed Sunni majority. Lebanese sources say these include Hezbollah's crack Unit 901 and that Hezbollah forces total as many as 4,000.

That's probably exaggerated, particularly since Hezbollah's leadership is reported to be divided over committing its best fighters to support a dictatorial regime, widely vilified across the Arab world, that's repeatedly used its Lebanese proxy as cannon fodder against Israel.

Hezbollah's leaders are said to be concerned that Assad will eventually be toppled and replaced by a regime of hard-line Sunnis who abhor Shiites in general, Hezbollah in particular.

Thus, as Tehran's behest, the Syrians are allowing Hezbollah to transfer its advanced arms, including missiles, from Syrian depots into the movement's heartland in the Bekaa Valley of northwestern Lebanon.

Israel's Jan. 30 airstrike against Syria reportedly destroyed a convoy of Russian-built SA-17 anti-aircraft missiles bound for the Bekaa.

More strikes may be in the cards, tightening pressure on Hezbollah as its strategic situation looks like it could soon deteriorate dramatically, leaving it cut off from its Iranian supply lines.

The group may find it needs those weapons not just to fight the ideological enemy Israel, but a resurgent al-Qaida and other Sunni militants in Lebanon itself in a widening Sunni-Shiite war that's spreading across the Muslim world.

Iranian National Security Director Saeed Jalili flew to Damascus right after the Israeli airstrike for talks with Assad and Hezbollah chiefs.

Israeli sources say he stressed it was vital Syrian continues transferring weapons to Hezbollah, possibly by dismantling them so they could be sneaked into Lebanon in less-conspicuous shipments by professional smuggling gangs who wouldn't alert Israeli surveillance systems.