A damaged mosque is pictured in the rebel-controlled area of Nashabyia town in Eastern Ghouta, Syria, April 13, 2016. REUTERS/Bassam Khabieh (Bassam Khabieh/Reuters)

A surge in fighting across Syria on Thursday signaled the apparent collapse of a landmark cease-fire that has been under mounting stress in recent days because of intensifying assaults by government forces and rebels.

The partial truce, which took effect in late February, represented a rare moment of agreement over the Syrian conflict between its most powerful outside players: Russia and the United States.

Although Moscow backs Syrian President Bashar al-Assad and Washington supports his opposition, the powers cajoled their Syrian allies into an agreement to cease hostilities to promote peace talks in Geneva that resumed Wednesday. The burst of fighting will almost certainly complicate those talks — now in their second round — and prolong a civil war that has killed more than 250,000 people and displaced millions.

“The more breaches of the truce we see, the more it shows that Assad does not want a political solution,” said the head of the opposition delegation in Geneva, Mohammed Alloush.

The opposition insists that a political solution requires Assad’s exit from power, but the Syrian leader and his allies have firmly rejected this.

A Russian soldier stands guard in the ancient city of Palmyra in the central Homs province, Syria, Thursday, April 14, 2016. Russian combat engineers are in Syria on a mission to clear mines in Palmyra, which has been recaptured from Islamic State militants. (AP Photo/Hassan Ammar) (Hassan Ammar/AP)

The United States has noted its concern in the jointly chaired U.S.-Russian task force that monitors the cease-fire, a State Department official said.

“In our conversations with Russian officials, we have proposed a series of measures to de-escalate the situation,” said the official, who spoke on the condition of anonymity because he was not authorized to discuss the subject publicly.

He did not provide details.

In Syria, rebel fighters reported heavy air raids in multiple areas including the northwestern province of Latakia and the central of province of Homs.

The Islamic State, meanwhile, claimed that it shot down a warplane Thursday in the southern province of Suwayda. On Twitter, the group said in a statement that its “air defense” system downed an aircraft as it took off from a military air base in the area.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said the government pilot survived the crash and eluded capture by Islamic State militants, who are known for summarily executing captives.

Rebel groups have long demanded heavier firepower from the West, including surface-to-air missiles, to counter the Syrian government’s air superiority. U.S. officials have declined to supply the weapons, fearing they could fall into the hands of extremists.

In the northern city of Aleppo, an assault by pro-government forces threatened to cut off the only supply road to rebel forces in the city. The capture of the Castello Road would deal a blow to the rebels in Aleppo, Syria’s largest city before the war, and to the wider rebellion against Assad.

According to Ameen al-Halabi, a pro-opposition media figure in the city, however, rebel fighters eventually drove government forces away from the Castello Road. “The clashes are continuing, but we regained positions that we briefly lost to the government,” he said.

The Syrian Observatory for Human Rights reported heavy airstrikes against rebel positions in Aleppo. It was unclear whether Russia carried out the air raids.

A government offensive in February — led by Russian airstrikes and thousands of Shiite militiamen from Iran, Lebanon and Iraq — nearly encircled Aleppo. If completely surrounded, the rebels in the city would likely face a prolonged and brutal siege.

Underscoring the complexity of fighting near the city, Islamic State militants mounted heavy assaults Thursday on rebels farther north, capturing several villages near the border with Turkey.

In recent weeks, the rebels have been driving the militants out of the area under the cover of Turkish artillery and airstrikes from the U.S.-led coalition that is combating the Islamic State in Syria and Iraq.

The Islamic State, also known as ISIS, is not a party to the cease-fire. Rebels, the U.S.-led coalition and pro-government forces — including Russia — have continued to target the group.

“The ISIS fighters attacked early in the morning, and they stormed into these villages by surprise,” said Jarrah, a rebel fighter in the area who asked that his full name not be used because of concerns for his safety.

The fighting appears to have aggravated an already dire humanitarian situation in the area. Hundreds, possibly thousands, of people from the affected villages were fleeing westward toward the rebel-held city of Azaz and makeshift refugee camps on the border with Turkey, aid workers said.

“We are seeing thousands of people arrive at the border, and more than a thousand families supported by the IRC at a displacement camp in Aleppo province have fled to Azaz and nearby villages,” Frank McManus, Turkey country director for the International Rescue Committee, said in a statement.

The rebel assaults on Islamic State positions near Syria’s northern border appear to be part of a new push by Turkey against the group. Analysts say Turkey also views the rebel forces as a buffer against Syrian Kurdish forces that have been capturing swaths of territory in the area.

Turkey claims that there are links between its own Kurdish separatists and Syria’s Kurdish militants.

Syrian Kurds unilaterally declared the creation of a federal region in northeastern Syria last month, despite warnings from Turkey and the United States. The issue has raised fears of a disintegration of Syria along ethnic and sectarian lines, an eventuality that would complicate efforts to end the fighting.