At least 27 people were killed and 30 others wounded on Sunday when a fuel tank atop a clinic rooftop in northern Syria exploded, the official Syrian TV said.The Maysaloun clinic in the city of Hasaka was set on fire after the fuel tank exploded, the report said, adding that most of the victims were children who were taking vaccines at the clinic when the blast occurred.Meanwhile, the opposition Syrian Observatory for Human Rights said the fire at the clinic broke out after a gas cylinder exploded, causing other oxygen cylinders to explode as well.It noted that many of the wounded were in critical conditions.