Shia officials say too much is at stake for military response in Lebanon despite Iranian embassy bombings that killed 23

Rescue workers had barely finished clearing the debris from outside the Iranian embassy in Beirut last week, when those who had been targeted gave their first decree.

There were to be no recriminations, Iranian and Hezbollah senior officials told their rank and file, hours after . Nor was there to be a military response, at least not in Lebanon.

The subdued aftermath of the first bomb attack on an embassy in the Lebanese capital in 30 years surprised many across the country, where dire warnings of unease spilling into outright chaos have been commonplace.

But Hezbollah officials, like their powerful patrons in Tehran, were staying true to their calculation of the past five years that their strategic gains in Lebanon, Baghdad and more recently Syria, would be compromised by striking back.

In the weeks leading up to Sunday's nuclear deal between Iran, the US and the UN, Hezbollah's leader, Hassan Nasrallah had offered a conciliatory approach to the west. A deal with the world's major powers was coveted just as much in Beirut as it was in Tehran, where leaders see a rapprochement with long-term foes as a consolidation of important progress in parts of the region.

Nasrallah belatedly acknowledged his group's hands-on role in the Syrian war in May, but he has since seemed anxious to insist that the group's battles need not be played out at home. The essence of the powerful militia's stance has been that while it needs to keep Sunni hardliners from travelling to Lebanon, it does not fear a homegrown uprising, carried out by Lebanese nationals.

Last week's bomb appears to have splintered that position; Lebanese officials believe at least one of the suicide bombers was Lebanese and that the group behind the attack is a local al-Qaida affiliate, the Abdullah Azzam Brigades.

In the eyes of Hezbollah's foes, the bombing has also laid bare the inherent dangers if Hezbollah continues to protect the Assad regime as it claws back lost ground in Syria.

"Their agenda is not Lebanese, it is Shia expansionism and it is protecting Assad to save its own skin," said a senior Saudi official. "Now they have bombers in their own backyard and are crying victim."

Lebanon's former prime minister, Saad Hariri, who has been in self-imposed exile between Jeddah and Paris since he was ousted 34 months ago, accused Hezbollah on Sunday of "bringing terrorism" to the country through its actions elsewhere.

Nevertheless, Hezbollah officials are holding firm. One senior member said on Friday: "We were reminded of [our] position, which has been not to let officials say anything throughout the Syria crisis. As well as that, there was to be no clear sign of anger or vengeance. All sides agree that it doesn't suit them for that to happen. And, in any event, we are winning."

Outside of the group's inner sanctum, Lebanese observers suggest the embassy bomb might make it more difficult for Hezbollah to continue to argue that its actions in Syria will safeguard, rather than imperil, the country.

Amin Hoteit, a retired major general in the Lebanese army said: "There will be some pressure from within certain quarters for Hezbollah to do something about this. There are problems here – sectarian problems – but it is not like it is outside of Lebanon. The parties on both sides of politics have a mutual interest in keeping things as calm as they can."

Hassan Jouneh, a Beirut-based international lawyer said it was becoming evermore difficult to position Lebanon as an "island in the regional storm".

"The regional situation is very complicated now," he said. "[This week] there were six mortars on the Saudi border. There are many attempts to stir trouble everywhere and it is going to take some managing. Lebanon is far from immune, but it is not centre stage."

Just east of the Sunni Muslim Lebanese border town of Arsal, momentum is building in one of the critical battles of the Syrian civil war, in which Hezbollah is again taking a lead role in directing the Syrian military around the town of Qalamoun. More than 15,000 refugees have fled the area for Arsal over the past week as Assad loyalists attempt to clear opposition groups, backed by global jihadists, from mountains and ravines standing between Damascus and a contiguous link to Homs to the north-west.

A regime victory here would give it a clear channel from the Syrian capital to the port of Tartous. It would also likely place Hezbollah in direct confrontation with the restive Sunni community of Arsal – a clash that would bring the war home, potentially jeopardising strategic gains in Syria.

"I'm telling you that if that happens, and it will, I am leaving Lebanon," said a 25-year-old student from the Hezbollah stronghold of Dahiyeh in south Beirut. "That is the most worrying thing for me. It will spread to Tripoli then. And Lebanon is finished."

How to contain crises within borders is also troubling Iraq, where a Sunni jihadist insurgency again rages against Shia interests and the government of Nouri al-Maliki. In Baghdad, as in Lebanon, officials say they will not be baited into retaliation, claiming that to fight back would be to lose recent gains.

In Iraq's case, the ousting of the Sunni powerbase of Saddam Hussein has directly empowered the country's majority Shia, which draws support from Iran.

Iraqi government spokesman Ali al-Mousawi said: "People have started to understand that there is a third party that wants to bring civil war to Iraq.

"If Assad manages to get rid of al-Qaida, then there is a strong chance of peace in Iraq, Lebanon and the rest of the region. But if Damascus falls under the spell of al-Qaida the whole region will be affected."

Hassan al-Zameli, a member of Iraqi Parliament's Defence Committee and loyalist of Shia hardliner, Moqtada al-Sadr, said: "There are ways to prevent a civil war in Iraq, but the government isn't taking the right action.

"Iran has a very big role in Iraq's stability, as does the US and Saudi Arabia. Syria is the key, if peace can be brought to it there will be stability in the region. There isn't a declared civil war here yet, but we are on the edge and if we don't stop it soon, this will lead to real damage for the whole region."