A gunman shot dead a filipino driver delivering rice for the United Nations in the restive southern Philippines, the agency said on Sunday.
Stephen Anderson, of the UN's World Food Programme (WFP), said the police were hunting the killer who struck outside Marawi city, where many armed groups including Islamic militants are known to operate.
"Right now we are treating this as an isolated incident," Mr Anderson told AFP, commenting on other press reports that blamed the separatist Moro Islamic Liberation Front (MILF) for the killing.
Mr Anderson said it was too early to blame any group for the attack.
The 12,000-strong MILF has been waging a decades-long rebellion for the establishment of an independent Islamic state in the southern third of the mostly Catholic country.
Mr Anderson said the driver, whose name he did not divulge, was locally hired and not a full-time staffer in the WFP.
He said the attack looked unprovoked although the motive has yet to be established.
The killing underscores the dangers humanitarian workers face in the south, where the rebellion is complicated by a proliferation of unlicensed firearms and large clans fighting over political turf.
In December 2008, a filipino aid worker was killed when unidentified gunman attacked a WFP convoy in the south, just three months after another convoy was stopped and its food cargo seized.