BEIRUT (AP) — The death toll from purported Russian airstrikes on the IS-held city of Raqqa in the past 24 hours has climbed to at least 55, mostly civilians, Syrian opposition activists and the Islamic State group said Sunday.

The extremist group has controlled the city in northern Syrian since 2013 and considers it the capital of its self-styled caliphate. It has recently become the focus of more intensified airstrikes. A Kurdish-led force supported by the U.S. and Russia has said that it is planning to liberate the city in the next months.

The alliance, known as the Syrian Democratic Forces, has had success fighting against IS in northern Syria, and expelled its militants from their stronghold in Hassakeh — the town of Shaddadeh — last month.

Intensified airstrikes on Raqqa were ongoing Sunday. The Britain-based Syrian Observatory for Human Rights, which documents the conflict through activists on the ground, said 55 people have been killed, including 13 children and a pregnant woman, in what it said were Russian airstrikes that hit residential areas over the past 24 hours.

The anti-Islamic State activist group Raqqa is Being Slaughtered Silently also said the airstrikes were by Russian warplanes and put the number of deaths at 60.

The IS-affiliated news agency Aamaq said 43 were killed and 60 wounded. It released a video that purports to show the "massacre committed by Russian aircraft in one of Raqqa's most congested streets." The video shows what appears to be dead babies as well as bombed out buildings, burning cars and other wreckage.

The activist groups on Saturday said the air raids struck near the national hospital, a former army base and other neighborhoods.

Russia has been conducting air raids in Syria since Sept. 30. Moscow began drawing down its military presence in Syria over the past week after President Vladimir Putin ordered a partial pullout of Russian aircraft and forces in support of indirect peace talks in Geneva.

On Thursday, Putin said Moscow would keep enough forces in Syria to continue the fight against the Islamic State group, the al-Qaida-affiliated Nusra Front and other extremist organizations.

The Islamic State group is not included in the truce that was brokered by Russia and the U.S. and went into effect on Feb. 27, leading to a drop of violence in Syria.

The air raids come at a time when IS has lost large swaths of territory in northern Syria, including in Raqqa province, in battles with the Syrian Democratic Forces.

To the south, Syrian troops are on the offensive in an attempt to capture the ancient town of Palmyra, which has been under IS control since May.

Opposition activists reported heavy airstrikes Sunday on the town that is home to some of the world's most precious archaeological sites. Several of those sites have already been damaged or destroyed by the extremists.

Syria's five-year civil war has killed more than 250,000 and displaced half the country's population.