* Mortar fire violates ceasefire agreement in Homs
* No immediate word on casualties, aid effort delayed
(Adds aid mission delayed)
BEIRUT, Feb 8 (Reuters) - United Nations-led efforts to deliver humanitarian aid to a rebel-held, besieged district of central Homs were delayed on Saturday by bombardment which Syria's warring foes each blamed on the other side.
The mortar fire on Saturday...