President Pranab Mukherjee on Friday said attacks on security forces "must be put down with a firm hand" as he "strongly condemned" the militant ambush on an army convoy in Manipur that left 18 soldiers dead.In his message to army chief, Gen. Dalbir Singh, the president, who is also the supreme commander of the armed forces, said: "Such random attacks on security forces performing their duty must be put down with a firm hand."I call upon all authorities concerned to work in close concert to bring the perpetrators of this heinous attack to justice and to maintain law and order in the state of Manipur."Expressing confidence that all possible aid was being provided to the bereaved families and medical assistance to the injured, the president offered his "most profound condolences" to the grieving families of the soldiers."Please convey my condolences to the families of the deceased personnel. I pray to the Almighty to give them the strength and fortitude to bear this irreparable loss. I also wish speedy recovery to the injured persons."At least 18 soldiers were killed and 11 others injured when NSCN-K militants ambushed their convoy in Manipur's Chandel district on Thursday.