Syria's air force fired missiles and dropped barrel bombs on rebel strongholds while opposition fighters attacked regime positions Sunday, flouting a U.N.-backed cease-fire that was supposed to quiet fighting over a long holiday weekend but never took hold.

The failure to push through a truce so limited in its ambitions — just four days — has been a sobering reflection of the international community's inability to ease 19 months of bloodshed in Syria. It also suggests that the stalemated civil war will drag on, threatening to draw in Syria's neighbors in this highly combustible region such as Turkey, Lebanon and Jordan.

"This conflict has now taken a dynamic of its own which should be worrying to everyone," said Salman Shaikh, director of the Brookings Doha Center think tank.

The U.N. tried to broker a halt to fighting over the four-day Eid al-Adha Muslim feast that began on Friday, one of the holiest times of the Islamic calendar. But the truce was violated almost immediately after it was supposed to take effect, the same fate other cease-fires in Syria have met.

Activists said at least 110 people were killed Sunday, a toll similar to previous daily casualty tolls. They include 16 who died in an airstrike on the village of al-Barra in northern Syria's mountainous Jabal al-Zawiya region.

The Observatory also reported a car bomb that exploded in a residential area in the Damascus neighborhood of Barzeh and wounded 15 people, but the target was not immediately clear.

Though Syria's death toll has topped 35,000, the bloodiest and most protracted crisis of the Arab Spring, the West has been wary of intervening. There is concern about sparking a wider conflagration because Syria borders Israel and is allied with Iran and the powerful Lebanese militant group Hezbollah.

There are already increasing incidents of the civil war spilling across borders.

Many in Lebanon blame Syria and Hezbollah for the Oct. 19 car bomb that killed the country's intelligence chief. The assassination stirred up deadly sectarian tensions in Lebanon, where Sunnis and Shiites are deeply divided over the Syrian civil war, raising the specter of renewed sectarian fighting.

Lebanon's two largest political coalitions have lined up on opposite sides of Syria's civil war. Hezbollah and its partners who dominate the government have stood by Assad's regime, while the Sunni-led opposition backs the rebels seeking to topple the Syrian government. Assad and many in his inner circle are Alawites — an offshoot of Shiite Islam and a minority in Syria — while the rebels come mostly from the country's Sunni majority.

Iraqi Shiites also increasingly fear a spillover from Syria. Iraqi authorities on Sunday forced an Iranian cargo plane heading to Syria to land for inspection in Baghdad to ensure it was not carrying weapons, the second such forced landing this month. The move appeared aimed at easing U.S. concerns that Iraq has become a route for shipments of Iranian military supplies that could help Assad battle rebels.

In Jordan, concern over stability was underlined last month, when its U.S., British and French allies quickly dispatched their military experts to help Jordanian commandos devise plans to shield the population in case of a chemical attack from neighboring Syria.

Turkey's support for the Syrian rebel movement is another point of tension, and Turkey has reinforced its border and fired into Syria on several occasions recently in response to shells that have landed from Syria inside Turkish territory.

The U.S. administration says it remains opposed to military action in Syria and politicians have been preoccupied this year with the presidential election, now a few weeks away.

On Sunday, Syrian warplanes struck the eastern Damascus suburbs of Arbeen, Harasta and Zamalka to try to drive out rebels, according to activists in those areas and the Britain-based Syrian Observatory for Human Rights, which compiles information from activists in Syria.