baghdad/beirut iraqi arm force saturday dislodge islamic state natural gas-rich border area syria accord military iranian-backed force fight syrian president bashar al-assad army simultaneously announce start offensive reach border area opposite side iraqi military statement say akashat desert region locate south euphrates river capture offensive announce early day attack akashat mean pave way recapture urban center euphrates valley include border post al-qaim say iranian-backed shi ite paramilitary force know popular mobilization sunni tribal fighters know tribal mobilization take part offensive add iraqi air force drop thousands leaflets overnight akashat well al-qaim towns ana rawa alongside euphrates tell militants surrender face death statement say two different campaign also advance islamic state position syrian side border syrian government force support russian air strike iran-backed militias u.s.-backed alliance kurdish arab fighters know syrian democratic force sdf alliance shi ite militias fight syrian army say launch assault reach al-bukamal syrian border town euphrates face al-qaim secure al-bukamal important iran ally two main cross syria north south control force ally u.s. secure land corridor across iraq could make easy iran ferry heavy weapons syria baghdad approve transfer shi ite-led iraqi government baghdad good relations tehran washington russian- u.s.-backed campaign islamic state syria mostly stay way side seek avoid conflict euphrates often act divide line senior assad aide week say syrian government ready fight u.s.-backed sdf recapture entire country cross-border caliphate declare islamic state 2014 effect collapse july u.s.-backed iraqi offensive capture mosul militants capital iraq towns border region syria hawija northern province close oil-rich city kirkuk urban center still islamic state control iraq group overrun third iraq 2014 sweep offensive allow militants grab hundreds millions dollars worth weaponry vehicles leave flee iraqi force iraqi force capture area syria border islamic state military