Syrian President Bashar al-Assad lost more territory on Tuesday to Islamist insurgents and Kurdish militias, bolstering Turkey’s push to create a rebel-controlled buffer zone along the two countries’ shared border.
The most striking opposition gains came in northwest Syria along Turkey’s frontier, according to representatives for various rebel factions and opposition activists in the area.
There, a Turkish-backed coalition of Islamist rebel groups captured more than a dozen villages, checkpoints and installations in a strategic area straddling Hama, Idlib and Latakia provinces that the Assad regime previously controlled, these people said.
The setbacks come two days after Mr. Assad said in a speech to his supporters that he was no longer able to defend many parts of the country. He said his depleted forces would instead focus on vital areas, a stark assessment for an authoritarian ruler who has held on to power during a 4½-year, multisided insurgency against him.
If the coalition succeeds in holding on to the advances, it would be a major blow to the regime and its supporters. The area constitutes the first line of defense of the Alawite heartland in western Syria and its center, Latakia city, and now even this area appears to be at risk.
The Islamist rebel gains could also put the U.S. in an awkward position. Washington has welcomed Turkey’s decision to commence airstrikes against the extremist group Islamic State inside Syria.
But some Islamist fighters whom Turkey supports but the U.S. deems enemies could gain sway in a planned buffer zone that Ankara and Washington jointly agreed last week to set up along the Turkish-Syrian border. The Islamist coalition, the Fateh Army, includes the al Qaeda affiliate Nusra Front, which is a U.S.-designated terror group. Likewise, the Kurdish militias, backed by the Washington in the fight against Islamic State but viewed as an enemy by Turkey, have added to the volatility.
U.S. officials said Tuesday they were fine-tuning plans for creation of the “ISIL-free zone” in northern Syria, a reference to Islamic State. Senior administration officials said they were open to working with a variety of moderate forces in Syria.
In the past year, the Assad regime has lost territory to Islamic State, Syrian rebels and Kurdish fighters. It is now almost completely shut out of eastern Syria and territories it still holds in the north, south and center are gradually shrinking. The regime and its allies are now mainly focused on bolstering defenses in Damascus and along the west coast.
Overnight Tuesday, rebels attacked regime positions with a barrage of heavy artillery and rockets and at least one suicide car bomber detonated his payload at the entrance of a village.
The Fateh Army posted videos and photographs on social-media websites showing what it said were their battle gains. In one image a section of the highway from Idlib to Latakia city on Syria’s west coast was littered with what looked like destroyed army vehicles and tanks. Another showed Islamist rebels hauling away crates of ammunition they said were seized from an outpost for regime forces in the village of Salat Zuhour in Idlib.
In one video, a rebel commander is shown in front of the Zeizoun power plant in the Ghab Plain, an agricultural area considered to be the fault line between the mostly Sunni Muslim plains to the east and the predominantly Alawite mountain region above the Mediterranean coast to the West.
Sunnis are leading the fight against Mr. Assad’s Shiite-linked, Alawite-dominated regime. Among them are the extremist Islamic State and more moderate Islamist and non-Islamist factions, as well as mostly secular Kurdish fighters. The rebels are backed by Turkey, Saudi Arabia and other Gulf nations.
On the other side, members of Mr. Assad’s own religious minority and Shiite militiamen from Lebanon are defending the regime, with financial and military support from Iran.
“The leader of the Assadist gang spoke the other day about how these positions are the gateway to Latakia, so today we destroyed this gate and we are going to enter the house as liberators praising God and rejoicing,” said a commander of Ahrar al-Sham, one of the largest Islamist factions taking part in the campaign, in a video released by the coalition.
“We are coming to you followers of Khosrow,” he added, referring to an ancient Persian king and using it in this context as a derogatory term to describe Alawites and Shiites allied to Iran.
Earlier this year, the same Islamist coalition captured the capital of Idlib province and several key towns and villages like Jisr al-Shughour.
On Tuesday, members of the Syrian-Kurdish People’s Defense Units, known as the YPG, took control of most of Hasakah city in northeast Syria following a monthlong battle with Islamic State militants.
The city was previously held by the Assad regime, and the Kurds stepped in to prevent Islamic State from seizing it. The Kurds now control most of it, with the regime relegated to a small position, according to residents and activists.
With the exception of occasional skirmishes, the Assad regime has so far avoided confrontation with the Kurds and the two sides have an arrangement whereby Kurds run their own affairs while the regime maintains some presence in their areas, according to Syrian Kurdish officials.
But this setup has frayed as Kurds have gained strength and territory and are now running their own de facto self-rule zone.
Syrian state media said regime forces repulsed the attacks by the Fateh Army coalition, adding that they killed more than 40 Nusra Front fighters. Opposition groups including the U.K.-based Syrian Observatory for Human Rights, which monitors the conflict, said the regime responded to its battlefield losses by unleashing airstrikes and barrel bomb attacks against several populated rebel-held areas in the northwest. Civilian casualties weren't immediately known.
— Mohmmad Nour al-Akraa in Beirut contributed to this article.