Syrian Forces and Hezbollah Push to Bolster Border Control

BEIRUT, Lebanon — Syrian government troops and Hezbollah fighters from Lebanon pushed into the Syrian border town of Zabadani on Sunday amid heavy airstrikes, continuing an offensive aimed at strengthening their control of routes between Lebanon and Syria.

Pushing insurgents out of the mountain town would continue advances aimed at cutting insurgent supply lines to Lebanon, and would also be a symbolic victory. Zabadani, a popular weekend getaway from Damascus before the war, was one of the first towns wrested from government control early in the revolt against President Bashar al-Assad that began in 2011.

Hezbollah, which has intervened on behalf of its ally, the Syrian government, is trying to cement control of the Lebanese frontier. It seeks to stop infiltration by insurgent groups, including the Nusra Front, Al Qaeda’s arm in Syria, which views Hezbollah’s Shiite base as apostates.

The Syrian government is seeking to secure the route from Damascus, the capital, to the Lebanese border and onward to Beirut. The road is a crucial link to the outside world that has taken on more importance as various insurgent groups have seized control over much of Syria’s borders with Turkey, Iraq, and Jordan.

To the northeast, in Raqqa, where the Islamic State’s self-declared caliphate holds sway, the United States-led coalition unleashed 18 airstrikes overnight that the American military said were aimed at disrupting the group’s freedom of movement. The strikes came along with 26 strikes in Iraq, where Islamic State fighters also hold large swaths of territory.

The military’s statement did not mention a reason for the timing of the unusually intense bombardment; the coalition normally conducts far fewer airstrikes in Syria on a daily basis. Local activists reported that six civilians were among 22 people killed.

Fighting across Syria on Sunday illustrated the complexity of the war. What began as an uprising against Mr. Assad devolved into a regional proxy war involving a wide array of combatants. Then, the Islamic State rose to power, battling rival insurgents and drawing direct intervention by the United States, in the form of airstrikes.

In the northeastern province of Hasaka, Islamic State militants continued to clash with government-allied forces and Kurdish fighters seeking an autonomous enclave. On the edge of Damascus, insurgents opposed to both the Islamic State and the Assad government launched a new offensive on government positions in the Jobar neighborhood — a once bustling region long reduced to rubble by shelling and airstrikes.

In the northern province of Aleppo, insurgents battling the government range from the Nusra Front to local rebels that have in the past received American support. They are fighting Mr. Assad’s army on one front, and the Islamic State on another.

And in neighboring Idlib, Nusra supporters were blaming the Islamic State for a suicide bombing that killed more than 20 Nusra fighters in a mosque as they prayed before breaking their Ramadan fast. Survivors said that a masked man entered the mosque with an explosives belt soon after a car passed blaring an Islamic State chant, “Soon you will see what amazes you,” a cleric affiliated with the Nusra Front said on Twitter.

In Zabadani, near the border, the government and Hezbollah appeared to have massed a large assault force, including members of the Syrian Army’s Fourth Division, seen as one of its most effective and loyal units.

Defending positions inside the town were fighters from the Nusra Front, the allied Islamist group Ahrar al-Sham, and local insurgents, according to an antigovernment activist there who gave only a first name, Firas, for his safety.

He said that insurgents had tried to launch a pre-emptive attack in recent days and had briefly seized some government checkpoints, but that loyalist forces had driven them back and entered the city from the west.

Dozens of barrel bombs — unguided explosives dropped from helicopters — had hit the city in recent days, Firas said. Images on Syrian state television and Hezbollah’s Al Manar channel showed pro-government soldiers advancing among cinder-block buildings and clouds of smoke rising over the mountains and a valley.