At least 13 people have been killed in a serious explosion at a petrol station in the southern Russian republic of Dagestan on Friday evening, according to Russian news reports. Two of the dead were children, and 11 people were injured in the blast, Russian news agency TASS reported, citing the regional health authority. A day of mourning has been declared for today, Saturday, in Dagestan. The cause of the explosion remains unknown and is under investigation. But an attack has been ruled out, and the Russian State Investigative Committee said no traces of explosives were found at the site. The…