A man and a child evacuate a rebel-held neighborhood of Aleppo, Syria, hit in an airstrike. (Ameer Alhalbi/AFP via Getty Images)

Fresh air raids and deadly clashes across Syria are threatening to unravel the country’s already fragile cease-fire, activists and rights monitors said Friday. The renewed fighting comes as peace talks brokered by the United Nations also are on the verge of collapse.

Airstrikes on rebel-held areas of Aleppo killed at least 19 people Friday, some of the deadliest raids since a cessation of hostilities took hold in late February, activists said. Government strikes also reportedly killed civilians in the nearby province of Idlib.

The reports come a day after Staffan de Mistura, a U.N. special envoy, told Radio Television Suisse in Geneva, the site of the talks, that the Syrian war has killed about 400,000 people, a figure far higher than previous U.N. estimates of 200,000 to 250,000.

In Geneva, the talks faltered this week as opposition delegates walked out in protest of what they said were Syrian regime violations of the cease-fire. On Friday, the head of the Syrian government’s delegation to the negotiations, Bashar Jaafari, said that he had met with de Mistura and that the talks would resume Monday. The opposition, represented by the High Negotiations Committee, has said that technical representatives will remain in Geneva to discuss humanitarian aid.

The U.N.-supported negotiations have stalled over the opposition’s demand that Syrian President Bashar al-Assad step down as a precondition for any political transition. The government delegation has said that Assad’s position as president is not negotiable.

With the cease-fire brokered by the United States and Russia under strain, the United States said this week that it was concerned about reports that Russia was rebuilding its military capabilities in Syria, where it began a military intervention last fall intended to boost Assad, a longtime ally. A few weeks ago, Russia said it was withdrawing its forces, but its troops and aircraft still appear to be participating in the fighting.

The Syrian Civil Defense force, a grass-roots volunteer unit that assists civilians in the aftermath of strikes, said Friday that its teams were reporting “streets littered with bodies” in such places as Aleppo, Syria’s largest city.

“We return to work with sadness and heavy hearts,” the group, also known as the White Helmets, posted Friday on Twitter. “Attacks are everywhere,” it said in a separate post.

Also on Friday, regime forces clashed with Kurdish fighters in the northeastern city of Qamishli for the third straight day. Syria’s ethnic Kurds have declared an autonomous zone in parts of northern Syria, raising tensions with government forces. Reports posted online by activists suggested that Kurdish forces from the People’s Protection Units, or YPG, managed to wrest control of several regime positions in Qamishli. Later, activists said a truce was declared.

The YPG has emerged as a key ally for the United States in its campaign against the Islamic State militant group in Syria. The YPG has also taken advantage of Russian airstrikes to fight rebels in areas it believes should be under Kurdish control.