It is what many international observers feared most.

Born out of the Arab Spring uprising, Syria's now full-blown civil war is spilling over its borders, threatening the wider region and drawing in players and backers from even further afield. Yesterday, there was yet greater evidence of how this conflict is increasingly driven on the battlefield by outside forces, as fighters from the Lebanese Shi'a Islamic militant group Hezbollah supported Syrian troops in a fierce offensive to seize more rebel territory in the town of Qusair, close to the border with Lebanon. Foreign Editor David Pratt takes stock of some key fracture lines and flashpoints in the fallout from the Syrian conflict

LEBANON

Perhaps more than any other neighbouring country, Lebanon is most vulnerable to any fallout from Syria's meltdown. In a recent survey by the respected US-based "fact tank" Pew Research Centre, more than nine in 10 Lebanese (95%) polled worry that Syria's violence may spill over into their nation. Such fears are shared by all the principal religious groups in Lebanon: the Christians (99%), Shia (95%) and Sunni (91%).

In sharing a border with Syria, Lebanon acts as both refugee sanctuary as well as a manpower and material resupply route for both Syrian rebel fighters and their sworn enemies, the pro-Assad Lebanese Shi'a Islamic militant group, Hezbollah.

Concern among ordinary Lebanese that the war is infecting their country is already borne out by the recent escalation in fighting in the city of Tripoli between factions supporting opposing sides in Syria's two-year civil war. Tripoli possesses a social fabric and history that makes it fertile ground for the long-awaited proxy war between enemies and allies of Syrian President Bashar al-Assad's regime.

The coastal city's Sunni Muslim majority supports the Sunni-led revolt in Syria, where Sunnis are also a majority.

But an Alawite enclave in Tripoli supports al-Assad, from the same sect, an offshoot of Shi'ite Islam. Assad has garnered support from many in minority groups like the Alawites.

Fear of Islamist radicals who are have joined rebel ranks in the fight against Assad's government has mobilised many in minority groups, including Alawites beyond Syria's borders. The launch of an offensive by Assad against the strategic Syrian town of Qusair, close to the Lebanese border, has added to the risks of the conflict spreading into Lebanon.

Hezbollah has helped Assad's forces in the assault there, inflaming Sunni-Shi'ite regional tensions that are already on the rise.

"I've never seen a day like this since the battle started," said Malek Ammar, an activist speaking from Qusair by Skype yesterday. "The shelling is so violent and heavy. It's like they're trying to destroy the city house by house."

Each of the opposing sides in Tripoli, meanwhile, accuses the other of using the city as a base to gather weapons and supplies headed into Syria and battlefields like Qusair. Residents said the recent spate of fighting in Tripoli has been the fiercest so far, including the use of heavy weapons such as advanced machine guns and mortar bombs.

Yesterday, the death toll in Tripoli reached 25 in the seventh straight day of clashes, while more than 250 people have been wounded in the fighting. It is for good reason that Tripoli has been called Lebanon's little Syria.

TURKEY

Of all of Syria's neighbours, Turkey has the most to gain by bringing the fighting to an end. Not only will that close down the possibility of a wider regional conflict, it will also resolve the growing refugee crisis which is putting an enormous strain on the country's resources.

In pursuit of that aim, Turkey's leader Tayyip Erdogan has been the most vocal regional leader in his condemnation of the fighting and the West's refusal to take a lead in finding a peaceful outcome.

His main recommendations so far have ranged from arming the Syrian rebels to creating safe havens and protected corridors for the thousands of Syrian refugees who are streaming out of the country every day, but he has found no takers in Washington.

Although President Barack Obama recognises Erdogan as a loyal friend and a key player in Middle East politics, he also has no wish to become involved in any plan which might involve US military assets.

There is also the memory of the largely unsuccessful attempt to protect the Kurds from Saddam Hussein's regime in Iraq using air power and how that led indirectly to the US-led invasion in Iraq in 2003. Obama has no wish to allow Turkey to persuade him to follow similar mission-creep in Syria.

All that has been agreed is that Turkey will permit US or Israeli warplanes to use their bases in the event of any increase in regional tensions.

That stalemate has not made life any easier for Erdogan, who is completely sincere in his desire to find an early resolution to the fighting in his neighbouring country.

Unless a deal is brokered soon, Turkish aid workers fear that they could be overwhelmed by the size and scope of a refugee problem which is already causing largely untold problems along the border, with reportedly one million Syrian refugees on the move.

An early opportunity comes next month with the "Friends of the Syrian People" meeting in Geneva, which will be attended by officials from Turkey, Saudi Arabia, the United Arab Emirates, Qatar, Egypt, the UK, France, Germany and Italy.

Turkish diplomats are already warning that the Syrian civil war is reaching a crisis point and that unless it is resolved soon, it will not just be refugees streaming over the border, but also armed rebel fighters intent on causing mischief.

ISRAEL

For more than two years, Israeli leaders have insisted they had no intention of intervening in the civil war raging in neighbouring Syria. All that changed in the last few weeks with a series of airstrikes aimed at stopping sophisticated weapons inside Syria from being transferred to Hezbollah, the Lebanese militia group.

As the US-based independent intelligence analysts group Stratfor pointed out, the Israeli airstrikes on Syria were predicated on two key factors. The first is that the Syrian regime is weakening and cannot control its territory and, by extension, its weapons stockpiles could fall into the hands of non-state actors such as Hezbollah and al-Qaeda. The second factor is that Israeli intelligence discovered a shipment of Iranian-made Fateh-110 short-range tactical ballistic missiles was being delivered to Hezbollah.

Logistically it is difficult to prevent advanced weapons systems from proliferating once a regime has lost control of them, so further strikes can be expected.

Israeli Prime Minister Benjamin Netanyahu has said that Israel is "preparing for every scenario" and that "we will act to ensure the security interest of Israel's citizens in the future as well".

But as a recent article in the New York Times highlighted, there is a general feeling among Israeli officials that the next time Israel strikes a weapons convoy, Syria's President Bashar al-Assad is much more likely to retaliate, given the increasingly strident statements coming out of Damascus lately.

Tensions have further heightened after Israeli soldiers exchanged fire with targets across the Syrian border in the Golan Heights, marking three consecutive days of cross-border fire. As far as Israel is concerned, Syrian regime loyalists and the rebel militias both threaten Israeli national security. Ironically, it is in Israel's interest to prolong the collapse of the al-Assad regime and to further the military stalemate.