* All deceptively quiet on Lebanon-Israel front
* Mutual deterrence staving off new war - Crisis Group
* Threats by Israel, Iran, Syria, Hezbollah unsettle region
By Alistair Lyon, Special Correspondent
BEIRUT, Aug 2 South Lebanon has been calm in the four years since Israel's 34-day war with Hezbollah, with both sides apparently reluctant to start a new conflict.
Yet the tranquillity, which has encouraged a tourism and real estate boom in Lebanon, may prove deceptive.
"Of course no one in the region is calling for war. But a pre-war mood is growing," wrote Volker Perthes, director of the German Institute for International and Security Affairs.
Israel, Hezbollah and its close allies Syria and Iran all say they espouse peace but are preparing for battle. Belligerent talk, even if intended to deter, is fuelling an ugly atmosphere.
Tension over Iran's disputed nuclear ambitions and a sense of despair about prospects for peace between Israel and Syria or the Palestinians also feed war fears in a region where U.S. power to influence events looks increasingly challenged.
The International Crisis Group (ICG) described the standoff between Israel and an "axis of resistance" as "exceptionally quiet and uniquely dangerous" in a report issued on Monday.
"The build-up in military forces and threat of an all-out war that would spare neither civilians nor civilian infrastructure, together with the worrisome prospect of its regionalisation, are effectively deterring all sides."
Israel, wary of any repeat of its 2006 failure to suppress Hezbollah, might also attack Syria next time. "The Israelis would want to send a tough message to the Syrians to cut off Hezbollah arms supply lines," said a senior diplomat in Beirut.
Just as Syria might get sucked into a new Israeli-Hezbollah round, Hezbollah would almost certainly find itself fighting Israel again in the event of any Israeli strike on Iran.
"Today, none of the parties can soberly contemplate the prospect of a conflict that would be uncontrolled, unprecedented and unscripted," said the ICG.
Yet Israel and its enemies have been talking incessantly about a coming showdown and pre-emptively blaming each other.
Iran's U.N. envoy Mohammad Khazaee said on Saturday that if Israel "commits the slightest aggression on Iranian territory, we will set fire to the entire war front and Tel Aviv".
President Mahmoud Ahmadinejad, while stating on Monday that "Iranians have never, ever favoured war", also mocked the notion of a U.S. or Israeli assault on Iranian nuclear sites.
For his part, Syrian President Bashar al-Assad used his country's army day on Sunday to declare that "the possibility of war is increasing" and to accuse Israel of blocking peace.
Israel refuses to rule out attacking Iran to stop it from breaking its own presumed nuclear monopoly in the Middle East.
Even the United States has acknowledged that it is planning for a possible war on Iran, which denies Western assertions that its nuclear programme has military as well as civilian purposes.
Admiral Michael Mullen, chairman of the U.S. joint chiefs of staff,asked by NBC's Meet The Press on Sunday if the military had a plan to attack Iran, replied: "We do."
Israel, which sees Hezbollah as a mere proxy of Iran, rather than as a group rooted in resistance to Israeli occupation of Lebanon, has multiplied its warnings to the Shi'ite guerrillas.
Israeli chief of staff Gabi Ashkenazi accused them in July of turning civilian areas in Lebanon into "surface-to-surface rocket villages" in readiness for attacks on Israel, although he said Hezbollah did not have an interest in picking a fight now.
Hezbollah leader Saeed Hassan Nasrallah has vowed to hit back in kind for any Israeli attack on civilian targets.
None of this means a new war in Lebanon, often a cockpit for regional antagonisms, is inevitable or imminent. Apart from mutual deterrence, other constraints are in play, the ICG said.
UNIFIL, the peacekeeping force in the south that was beefed up after the 2006 war, acts as a buffer between Israel and Hezbollah, even if both sides accuse each other of violating the Security Council resolution which modified its mandate.
Hezbollah, now an integral part of Lebanon's unity cabinet, has a stake in restraint. Israel's government has also avoided escalation since 2006, responding in a measured way to a few cross-border rocket salvoes not thought to be Hezbollah's work.
"U.S. President Barack Obama, likewise, far from the one-time dream of a new Middle East harboured by his predecessor, has no appetite for a conflagration that would jeopardise his peace efforts and attempts to restore U.S. credibility in the region," the ICG report argued.
But it said only Israeli-Syrian and Israeli-Lebanese peace talks could address the political roots of the crisis. Short of that, it urged international efforts to enhance communications between the parties, defuse tensions and avoid costly missteps.
"Beneath the surface, tensions are mounting with no obvious safety valve," the conflict-prevention group said. (Editing by Jon Boyle)