Deliveries of food aid to Somalia by sea have been halted, after an attempt by pirates to seize a ship chartered by the UN food relief agency.
Pirate attacks have risen since the Union of Islamic Courts was ousted
The head of the World Food Programme said their programme to feed one million Somalis is under threat.
The WFP-chartered vessel was attacked on Saturday off the Somali port of Merka after it had just delivered 4,000 tons of food.
The pirates killed a guard who was among a group sent to intercept them.
Following the attack, the agents for a ship loaded with food in Mombasa in Kenya have refused to allow the vessel to set sail for Somalia until they are given an armed escort.
"We are not taking any risks after being victims four times. We planned to go to Somali this week but following Saturday's incident our ship will not sail," Karim Kudrat who owns MV Rozen that was hijacked in Puntland said.
Shipping is the main and fastest way of getting food relief into Somalia.
In a statement the WFP executive director, Josette Sheeran, said: "We urge key nations to do their utmost to address the plague of piracy, which is now threatening our ability to feed one million Somalis."
"This attack underscores the growing problem of piracy off Somalia which, if unresolved, will sever the main artery of food assistance to the country and to the people who rely on it for their survival," she added.
The ship was on its way to the port of Dar es Salaam in Tanzania after delivering food aid to Merka, south of Mogadishu.
Last year the WFP had to suspend aid deliveries for weeks after the hijacking of two ships it had contracted.
"WFP is very saddened and alarmed by the death of the guard, who showed great courage while the ship came under attack. We send our condolences to his family," Ms Sheeran said.
Piracy has been rife off Somalia since the country slid into lawlessness in the 1990's although during the brief six-month rule of the Union of Islamic Courts last year, attacks decreased.
Since the Islamists were ousted by the Ethiopian and transitional government forces, there have been at least five cases of ships being seized.
Three vessels were boarded by pirates last week.
Two fishing trawlers from Korea and one from Taiwan are still being held somewhere on the Somali coast.
Many pirates claim to be coastguards, protecting waters against illegal fishing and the dumping of toxic waste.
Maritime security experts who have been monitoring piracy along the Somali coast say investigations have revealed that the pirates are being backed by key figures within the autonomous administration in Puntland, where most negotiations for the release of hijacked ships are done.
"We have evidence that the pirates have a main contact in Puntland and it's up to the interim government in Somalia to track and arrest the contact for taking part in an illegal syndicate," the maritime expert told the BBC News website on condition of anonymity.