The call comes amid criticisim that Somalia is not receiving the same attention from the UN as the crisis in Sudan's war-torn Darfur region.
Hundreds of thousands of Somalis are believed to be on the verge of starvation as Ethiopian troops and the forces of the transitional government battle tribal and Union of Islamic Courts fighters.
Speaking to Al Jazeera, Holmes said he was trying desperately to visit Somalia on his upcoming trip to the horn of Africa.
"That window of opportunity seems to be closed"
John Holmes, UN humanitarian relief coordinator
"A situation was created by the Ethiopian invasion and intervention and putting in [a] transitional government seemed to be a window of opportunity … to restore peace," he said in New York.
"That window of opportunity seems to be closed… or is closing."
Holmes said the political way forward has to come from Somalia's transitional government as well as its neighbour, Ethiopea, coupled with serious pressure from the international community.
The UN appears divided over the handling of the Somalian crisis.
While the Security Council prepares contingency plans for a Somalia peacekeeping force the UN chief says a political solution must first be reached.
Critics have said the UN, and the United States, have listed Darfur as their top priority even though the humanitarian crisis in Somalia was much worse.
|About 600,000 people have so far fled|
violent fighting in Mogadishu [AFP]
The UN High Commissioner for Refugees said onTuesday that about 600,000 people have fled the war-ridden Somali capital, Mogadishu, with about one-third of them fleeing heavy fighting in recent weeks.
"Families continue to lack proper shelter and consistently resort to using any material, mostly plastic bags and rags, to patch up their 'tukuls' or flimsy dome-shaped shelters," the UNCHR said in a statement.
Numerous bids to stabilise Somalia have been undermined by the bloody clashes between Somalian clans and power struggles that intensified after dictator Mohamed Siad Barre was overthrown in 1991.