BEIRUT — Syrian government warplanes carried out a series of airstrikes Tuesday on the de-facto capital of the extremist Islamic State group, killing at least 36 people, activists said.

The Britain-based Syrian Observatory for Human Rights said some of the air raids struck a popular market near a museum and an industrial neighborhood in the city of Raqqa along the Euphrates River in northeastern Syria.

It put the death toll at 36. The Local Coordination Committees said the strikes killed at least 50 people. Another Raqqa-based collective called Raqqa is Being Silently Slaughtered said it documented over 70 deaths. Such discrepancies are normal in the immediate aftermath of attacks in Syria.

A militant video posted online showed medics bundle four bloody bodies into the back of an ambulance amid shouts of "God is Great." In the background, a fire truck tries to douse several burning cars as gray smoke rises into the sky.

The video appeared genuine and corresponded to Associated Press reporting on the strikes.

The Syrian government as well as the U.S.-led coalition frequently bomb Islamic State group targets in Raqqa. It was not immediately clear what prompted Tuesday's unusually intense strikes.

An activist who uses the name Abu Ibrahim al-Raqqawi said nine airstrikes took place within half an hour. All but one struck civilian neighborhoods in the center of the city, he said, with one knocking off the minaret of a mosque.

"This is one of the ugliest regime massacres in Raqqa to date," said the Moscow-based activist, who oversees Raqqa Is Being Silently Slaughtered network.

In Iraq, the Islamic State group blew up the al-Nasir convent in the northern city of Mosul, which has been controlled by the militants since June. Its resident nuns fled the city along with most of Mosul's remaining Christians when militants overran the city.

The Islamic State group has frequently targeted Christians by bombing their churches and killing clergymen, as well as religious minorities across Iraq's north.

Associated Press writer Sameer N. Yacoub in Baghdad contributed to this report.