A rocket attack in Damascus on Sunday that Syria blamed on Israel hit an installation where Iranian officials were meeting to advance programs to develop drone or missile capabilities of Tehran's allies in Syria, sources told Reuters in an exclusive report published on Wednesday. Iran has been a major backer of President Bashar al-Assad during Syria's nearly 12-year conflict. Its support for Damascus and the Lebanese group Hezbollah has drawn regular Israeli air strikes meant to curb Tehran's extraterritorial military power, according to the Reuters report. A source close to the Syrian government with knowledge of Sunday's strike and its target told Reuters it hit a gathering of Syrian and Iranian technical experts in drone manufacturing, though he said no top-level Iranian was killed. “The strike hit the center where they were meeting as well as an apartment in a residential building. One Syrian engineer and one Iranian official - not high-ranking - were killed,” the source told Reuters. The Syrian Observatory for Human Rights, a UK-based war monitor, had reported that the Israeli air strike on Damascus had killed 15 people that included nine Syrians. Three Syrian officers were killed in the strike. The overnight strike cratered a road and wrecked the adjacent 10-story building in the Kafr Sousa district, which is home to senior state officials and Syrian intelligence headquarters, said the Britain-based Observatory. Other missiles overnight hit a warehouse used by pro-regime Iranian and Hezbollah fighters near Damascus, said the Observatory, which relies on a wide network of sources inside Syria. This rocket strike, along with others that Israel says target infrastructure of Syria's military and its allies, reflect an escalation of what has been a low-intensity conflict aimed at slowing down Iran's growing entrenchment in Syria, according to Israeli military experts. Syrian state media said at the time that Israel had carried out air strikes shortly after midnight on Sunday against several areas of the Syrian capital, causing five deaths and 15 injuries including civilians. An Israeli military official declined to confirm or deny that Israel was behind the attack but said some of the casualties were caused by errant Syrian anti-aircraft fire. The US and Israel have been increasingly concerned about Iran’s drone manufacturing, and the possibility it would pass on those capabilities to regional proxies such as the heavily armed Hezbollah. Last week US forces shot down what they said was an Iranian-made drone flying over a base hosting US personnel in northeastern Syria.