BEIRUT (AP) — Government forces and rebels clashed Sunday across northern and western Syria, imperiling a monthlong cease-fire ahead of peace talks in Geneva, while airstrikes pounded the Islamic State group's de facto capital of Raqqa, killing dozens.
Al-Qaida's Syrian affiliate, the Nusra Front, is playing a leading role on the side of the insurgents, according to the Britain-based Syrian Observatory for Human Rights, a monitoring group with a network of informers inside Syria.
The fighting follows weeks of sporadic government airstrikes, culminating with a raid that killed 33 civilians outside the capital of Damascus on March 31 that tested the durability of a U.S.- and Russia-brokered "cessation of hostilities" that took effect in late February.
The Nusra Front and the Islamic State group are excluded from the cease-fire, which had brought relative calm to much of Syria for the first time in the 5-year-old civil war between forces loyal to President Bashar Assad and those trying to topple him.
Observatory head Rami Abdurrahman said fighting was intensifying around the northern city of Aleppo and "definitely" threatens the cease-fire.
U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov spoke by phone on the need for greater cooperation to strengthen the truce, Russia's Foreign Ministry said.
They also discussed efforts to fight the Islamic State group, the Nusra Front and other terrorist organizations, agreeing to take additional measures to halt the flow of militants and weapons from abroad, the ministry said.
The diplomats also expressed support for the efforts of the U.N. envoy to Syria, Staffan de Mistura, to convene the next round of talks in Geneva this week between the Syrian government and the opposition, Lavrov's office said. De Mistura is in Damascus to prepare for the talks and was expected to meet Syrian Foreign Minister Walid al-Moallem.
But expectations of progress have been dampened by the surge in fighting, the continued obstruction to humanitarian access and the staunch positions by the government, the opposition and their international backers.
A leading member of the opposition High Negotiations Committee, which is scheduled to participate in the indirect talks, warned that the cease-fire is on "the brink of collapse."
"The government has resumed using barrel bombs," Bassma Kodmani said in an interview with the French weekly Le Journal du Dimanche, referring to the crude and indiscriminate weapons that observers say have killed thousands of civilians in opposition-held areas.
Syrian Prime Minister Wael Nader al-Halki told a Russian parliamentary delegation in Damascus that Syrian troops and the Russian air force are preparing a joint operation to retake al-Eis, Russia's Tass news agency reported.
In Iran, a top adviser to that country's Supreme Leader said Assad's removal from power is a "red line" for Tehran. Iran has been a key Assad ally throughout the conflict. State-run press TV quoted Ali Akbar Velayati as saying that Iran believes Assad's government "should remain in power until the end of the presidency term."
In 2014, Assad was re-elected to a seven-year term in balloting held in territory controlled by his forces. The vote was widely condemned and boycotted by the opposition.
Jan Egeland, head of the U.N. task force on humanitarian access in Syria, said Friday he was "disappointed and disheartened" by the obstructions to humanitarian access. He said the government blocked four convoys from hard-to-reach areas while rebels blocked a fifth in recent days, affecting 287,000 in need of aid.
Insurgents advanced on government positions on the periphery of Latakia province in the northwest, an Assad stronghold, and battles raged south of Aleppo, where opposition forces seized the strategic village of al-Eis eight days ago, killing dozens of government troops and allied fighters from the Lebanese militant group Hezbollah.
In northwestern Syria, a coalition of Islamist and non-Islamist factions led by the jihadist Ahrar al-Sham group and the Nusra Front seized the village of Baydaa, while Ahrar al-Sham, the Nusra Front, and other jihadist groups battled government forces on the Sahl al-Ghab plains, north of the city of Hama.
A Lebanese TV station close to the Syrian government, al-Mayadeen, said government forces repelled the offensive on Sahl al-Ghab, while the Observatory said there are reports of several casualties as the fighting continued to rage.
The Observatory said at least 35 fighters died in weekend clashes south of Aleppo.
In the north, the IS group launched an offensive along the border with Turkey, seizing two villages near al-Rai, a frontier town it lost Friday to the Western-backed Free Syrian Army. IS militants seized Sheikh Reeh and al-Bal on Sunday, the Observatory said.
The extremists set off seven bombs around nearby Marea and other villages and towns, according to the activist-run Azaz Media Center, based in a northern town of the same name, which called it the fiercest IS offensive in a year.
The IS-linked Aamaq News Agency announced the capture of the two villages and said a suicide operation targeted opposition-held Kafr Shush.
Abdurrahman said IS was seeking to protect its stronghold in nearby Dabiq. The town has symbolic importance to the extremists, who believe medieval prophecies that it will be the site of a doomsday battle against infidels.
Airstrikes around Raqqa, the so-called capital of the group's self-styled caliphate, killed dozens of militants and civilians, monitoring groups said.
The group Raqqa Is Being Slaughtered Silently, made up of activists who still have contacts in the city, said a notorious IS judge, Fawaz al-Hassan, was among those killed. Al-Hassan, also known as Abu Ali al-Shari, had been expelled from IS for his extreme legal interpretations but was later brought back into the fold to frighten potential defectors, the group said.
Among the dead were three leaders and 21 other militants, as well as eight civilians, the Observatory said, while Raqqa is Being Slaughtered Silently said 20 civilians were killed and more than 50 wounded.
Syria, Russia and a U.S.-led international coalition have been bombing IS for months. It was unclear who was behind Sunday's airstrikes.
Associated Press writers Sarah El Deeb in Cairo and Lynn Berry in Moscow contributed to this report.