By Agence France-Presse Islamic State group jihadists have killed 27 pro-Damascus fighters in the Syrian desert, in what a monitoring group described Saturday as their deadliest operation since the fall of the "caliphate". A Syrian soldier inspects the wreckage of a building described as part of the Scientific Studies and Research Centre (SSRC) compound in the Barzeh district north of Damascus, during a press tour organized by the Syrian government after US-led strikes. (AFP photo / MANILA BULLETIN FILE PHOTO) The Britain-based Syrian Observatory for Human Rights said four senior Syrian army officers were among the troops and allied militiamen killed in the desert east of Homs province over the past 48 hours. The Amaq propaganda arm of IS, which lost the last vestige of its "caliphate" to Kurdish-led forces last month but retains desert and mountain hideouts in both Syria and Iraq, said its fighters carried out the operation. Observatory chief Rami Abdel Rahman described it as the "biggest attack and the highest death toll among regime forces since the caliphate was declared defeated". Six IS fighters were killed in the clashes, the Observatory said. Amaq said that IS had launched its assault on Thursday evening after government forces tried to track down its fighters. The vast Syrian desert, known in Arabic as the Badia, stretches all the way from the capital Damascus and the cities of Homs and Hama to its north to the Euphrates Valley near the Iraq border, where IS made a desperate last stand for its final pocket of territory.