BEIRUT (Reuters) - Warplanes bombed the eastern Syrian city of Raqqa, stronghold of Islamic State insurgents, on Thursday, hitting some of the group's offices and killing at least four civilians in the vicinity, residents and a monitor group said.

A resident in the city said there were at least 17 air strikes on the heart of Raqqa, and electricity had gone off in the city.

The UK-based Syrian Observatory for Human Rights, which monitors Syria's civil war through a wide network of sources inside the country, said the air strikes hit several locations used by Islamic State, including the perimeter of Raqqa's Governorate building.

At least four civilians were killed, it said.

It was not immediately clear who bombed Raqqa, which in the past few months has been targeted by a U.S.-led air strike coalition and by Russians and Syrian government warplanes.

(Reporting by Mariam Karouny and Suleiman al-Khalidi; Editing by Mark Heinrich)