At least 15 civilians including four children were killed in airstrikes in Syria's al-Raqqa city, a UK-based war monitoring group reported on Saturday.The Syrian Observatory for Human Rights (SOHR) said the bombing was carried out in the district of Hunaida, Efe news reported.SOHR added that the death toll could increase.According to the SOHR, nearly 220 civilians, including 36 minors, have been killed since March 1 in bombings targeting al-Raqqa.Al-Raqqa is the main stronghold of the Islamic State terror organization in Syria.