* Ambush led by Shi'ite Muslim Hezbollah - Observatory
* Most of the dead foreign Islamists - state media
* Underlines complexity of increasingly sectarian conflict
By Stephen Kalin
BEIRUT, Feb 26 (Reuters) - Forces loyal to Syria's president killed at least 175 Islamist rebel fighters, most of them foreigners, in an ambush in insurgent-held eastern outskirts of Damascus, state ...