Damascus, Aug 13 : The death toll from the Islamic State (IS) attack on a Syrian military bus on rose to 33, a war monitor reported. The IS militants attacked the bus in the desert region of al-Mayadeen city in the eastern province of Deir al-Zour before daybreak on Friday, said the Syrian Observatory for Human Rights on Saturday. The UK-based watchdog group added the IS militants planned the ambush and opened fire on the bus with light and medium-sized weapons, Xinhua news agency reported. The Syrian army was on alert in the desert region, searching for soldiers who had gone missing, while the attackers disappeared into the desert, as reported by the observatory. Meanwhile, video footage believed to have been shot by one of the killed soldiers' mobile phones circulated online, showing the soldiers joyfully singing before the attack. The Syrian military confirmed the incident in a statement without giving the death toll. As the IS has intensified its attacks in the desert region, 420 people, mainly soldiers, have been killed since the beginning of 2023 by the IS in the area, according to the war monitor. /IANS