BEIRUT - Warring sides exchanged rocket and gunfire north of the Syrian city of Homs overnight, hours after a Russia-backed truce took effect, a war monitor said on Friday, while heavy rocket fire also marred a similar deal east of the capital Damascus.Russia, an ally of the Syrian government, said on Thursday its defense ministry and Syria's opposition had agreed to set up a "de-escalation" zone in the rebel-held countryside north of government-held Homs.After an initial few hours of calm, the rebels and government forces and their allies began to target each other's territory. The monitor, the Britain-based Syrian Observatory, said it had so far not received reports of any deaths.The Russia-backed truce was similar to a de-escalation deal worked out in July for the besieged Eastern Ghouta rebel enclave east of Damascus.Despite the deal and some reduction in violence, air strikes, rockets and exchanges of fire have continued to hit Eastern Ghouta.The Syrian Observatory said since the Eastern Ghouta truce was declared on July 22 it had recorded at least 25 civilian deaths, including seven children, and dozens of injuries. Russia said it had deployed its military police in Eastern Ghouta in July to try to enforce the de-escalation zone.Eastern Ghouta, the only major rebel-held area near the capital, has been blockaded by Syrian government forces since 2013. It has shrunk considerably in size over the past year as the Russia-backed Syrian army has taken control of other rebel-held areas around Damascus. The Observatory said on Friday around 70 rockets had fallen in 24 hours on Eastern Ghouta in the heaviest bombing since the de-escalation zone was declared.Several attempts at a lasting ceasefire in western Syria, where rebels have lost ground to government forces and their allies, have collapsed with both sides blaming the other for outbreaks of violence.