No-one took seriously the claims by “Battalions of Ayesha” — a fictional terror group — that it had planted the massive car-bomb that ripped through a south Beirut neighbourhood last Thursday, killing at least 27 people and wounding over 200.

While some Lebanese politicians were quick to blame Israel for the blast — after all, the bomb had gone off near Hizbollah offices — but since the attack has not been connected to a specific target or individual, there is little question as to the motivation for the attack.

The explosion in the Dahiya quarter was clearly intended to cause as many Shia casualties as possible, serving as yet another episode in the increasingly bloody sectarian conflict that the Syrian civil war has become.

“There is no question that this was carried out by some affiliate of al-Qaeda,” said an Israeli defence source. “Probably the Al-Nusrah front or some other Sunni group. Nasrallah has bitten off more than he can chew and now his intervention in Syria is coming back to hit him in the face.”

The message certainly was not lost on Hizbollah chief Sheikh Hassan Nasrallah, who said in a speech on Hizbollah’s Al-Manar channel on Friday: “I will go myself to Syria if it is necessary in the battle against the takfiris [Sunni radicals],” but he also warned his followers not to respond blindly and risk dragging Lebanon into the Syrian war.

By responding with both aggression and caution, Nasrallah exhibited the pressure he is under, both from his Iranian patrons and from Lebanese rivals.

It is still unclear whether the decision to involve Hizbollah heavily in the fighting in Syria on the side of President Bashar Assad’s beleaguered forces was made on Nasrallah’s initiative or at the urging of Iran, anxious to save its ally. Either way, this policy has largely backfired, despite some initial success on the battlefield.

Hizbollah remains a central player in Lebanese politics, claiming to be “the resistance” to Israel, which uses to justify holding on to its paramilitary force, with tens of thousands of missiles aimed at Israel. However, it is getting harder for the terror organisation to maintain that position — not only has it been sucked into the Syrian quagmire but, as a result, it is now causing new terror attacks on Lebanese soil.

On Sunday, Arab media reported that Ali Hussam Nasser, a senior Hizbollah field commander, had been killed in the fighting near Damascus. Hundreds of the organisation’s members have been lost in Syria; it has already sustained more casualties than it did against Israel in the 2006 Second Lebanon War.

Sheikh Ahmad al-Assir, who has become one of the most prominent Sunni leaders in Lebanon opposing Hizbollah, blamed the Shia movement, saying that the Beirut bombing was “a result of their crimes in Syria”.