"One of our national staff in Jaffna was murdered this morning," said Charles Macfadden, head of the group's Sri Lanka mission.
"We know nothing. He was on his way to work, we understand he dropped (off) his wife, stopped and had a chat with someone and someone came behind him," he added. "We have no information as to why or wherefore."
The killing comes weeks after two Sri Lanka Red Cross volunteers were found murdered after they were abducted from the capital by men in plain clothes who identified themselves as policemen. Police denied any involvement.
The military had no immediate details on the killing, which took place on a peninsula where an estimated 50,000 troops are stationed and where Tamil Tiger rebels continue to mount sporadic attacks amid a new chapter in a two-decade civil war.
The international community has repeatedly voiced concerns about rights abuses blamed on the Tigers as well as elements of the Sri Lankan military.
Nordic truce monitors suspect elements of the military were behind the execution-style murder of 17 local staff of aid agency Action Contre la Faim in the island's east last year, the worst attack against humanitarian workers since the 2003 bombing of the United Nations headquarters in Iraq.