(CNN) -- Rocket attacks in Lebanon. Car bombs in Turkey. Israeli airstrikes in Syria.
In the two-plus years since President Bashar al-Assad's crackdown on "Arab Spring" demonstrations, observers say the civil war that grew out of it has now become a multi-sided conflict that threatens to set the wider Middle East ablaze.
"The Syrian conflict is no longer an internal struggle between Assad and the internal opposition," said Fawaz Gerges, director of the Middle East Center at the London School of Economics. "It's an open-ended war by proxy -- Iran, Hezbollah and Syria, Turkey, Qatar and Saudi Arabia, plus Russia and the United States."
In the meantime, he said, Syrian society is disintegrating. And after more than 70,000 deaths inside the country, the conflict is increasingly jumping the borders.
The past few weeks have seen a pair of car bombs kill dozens of people in a town that has welcomed some of the 300,000-plus Syrian refugees who have fled to Turkey.
Turkish officials said the bombings were carried out by members of a former Marxist terror group with ties to Syria's intelligence services; Syria denied responsibility, but said Turkey ,a NATO ally, had been helping "terrorists" get weapons and money.
Meanwhile, the Persian Gulf monarchies of Saudi Arabia and Qatar have backed Sunni rebel factions against al-Assad, a member of the Shiite offshoot Alawite sect. The European Union is lifting an arms embargo on Syria after Britain and France refused to agree to an extension.
But on the battlefield, the momentum that appeared to be on the rebel side earlier this year now seems to have shifted to al-Assad, said Robin Wright, a Middle East analyst at the Woodrow Wilson Center in Washington.
"That will go back and forth," Wright told CNN. "The tragedy of this is the inflow of weapons just means more people are going to be killed, and there doesn't seem to be a military outcome likely on either side anytime soon."
France says Hezbollah, the powerful Lebanese Shiite militia backed by Iran and Syria, has dispatched up to 4,000 fighters to Syria to bolster al-Assad's forces. Gerges said those fighters have "already produced major results," particularly in the ongoing battle for the strategically located border town of Qusayr.
Rocket attacks have struck Shiite towns inside Lebanon, where a fragile sectarian and political balance has held since the end of a civil war that wracked the country from 1975 to 1990. And three Lebanese soldiers were killed by unidentified gunmen who opened fire on their checkpoint this week, Lebanon's national news agency reported.
Walid Jumblatt, a veteran Lebanese political leader and a former Syrian ally, contends the conflict threatens to reopen Lebanon's old wounds. But he said confronting Hezbollah over its involvement "will just lead us to the sectarian warfare that is starting in Iraq, in Syria and might spread to Lebanon."
"I'm more concerned about the stability of my country," Jumblatt, the leader of Lebanon's Druze minority, told CNN's "Amanpour."
Fighting has also spilled over into Iraq, with jihadist groups on both sides of the border growing in strength, Western counterterrorism officials warned in March.
Fighters from the Islamic State of Iraq, the al Qaeda affilliate that has bedeviled Baghdad for years, said it had killed at least 40 Syrians in an ambush on a Syrian convoy inside Iraq. The troops were being escorted by Iraqi forces to the only border post the Syrian government still controlled.
"The increasing number of foreign fighters crossing Syria's borders to support one side or the other is further fueling the sectarian violence and the situation is beginning to show worrying signs of destabilizing the region as a whole," Navi Pillay, the U.N. High Commissioner for Human Rights, warned Wednesday.
Then there's Israel, which is believed to have conducted at least two airstrikes inside Syria to prevent Syrian forces from transferring advanced missiles to Hezbollah.
Israel fought a month-long war with Hezbollah in 2006 and never signed a peace agreement with Syria after the 1948, 1967 and 1973 Mideast wars.
Syria said Thursday that Russia, its most powerful ally, will deliver on a 2010 purchase of advanced anti-aircraft missiles. Moscow has defended the deal, saying it falls within international law and that the missiles aren't designed for use against civilians.
Gerges said the deal is a strong Russian signal to the West: "Stay away from Syria."
"Russia is the backbone of the Assad regime. It has provided them with arms. It has provided them with political support. It has used its veto twice in the (U.N.) Security Council. It has gone to great lengths to prevent any kind of military intervention in Syria," he said.
The United States has provided non-lethal aid and political support to the Syrian opposition, but the Obama administration has resisted calls to provide military aid to the rebels.
At the same time, Washington is trying to work with Russia to coax the opposition and the government to peace talks, concerned about "a region-wide conflict," Gerges said.
"That's why they have intensified their diplomacy to rescue Syria from really all-out destruction and also rescue the entire region from a region-wide conflict where American and international peace and security are really at stake," he said.
But the opposition Syrian National Coalition said Thursday that it wouldn't take part "when Syrians are constantly being hammered by the Assad regime with the help of outside forces," as George Sabra, its acting chairman, put it.
The opposition remains split along secular and sectarian, military and political lines. Those divisions have been "a real obstacle" to negotiations, Wright said.
"Both the United States and Russia have agreed that diplomacy is necessary, and they haven't been able to agree on that, even, for a long time," she said.