Navy vessels have had to rescue ships and yachts from recent pirate attacks
The UN World Food Programme has warned it may have to cut food aid to Somalia if it does not receive new naval protection against pirates.
French, Danish and Dutch frigates have protected deliveries for more than seven months during a surge in piracy, the WFP said.
But a Dutch frigate was scheduled to finish escort duties on Wednesday.
As many as two million people could be affected if the shipments cease, the WFP said.
The agency said shipping companies were reluctant to sail unescorted to Somalia, and it had no offers to take over escort duties from the Dutch navy.
About 80% of WFP aid to Somalia arrives by sea.
"Without escorts, our whole maritime supply route will be threatened," said Peter Goossens, WFP's country director for Somalia.
"If relief shipments slow down, we could face a major catastrophe," he said.
The UN has warned that the food crisis in Somalia is dramatically worsening, with the number of people in need of emergency food aid likely to rise to 3.5 million in the coming months.
A WFP spokesman told the BBC that if Somalia received no humanitarian aid, the country could face a famine similar to the one that killed hundreds of thousands 15 years ago.
Among the contributing factors are bad harvests, conflict, hyperinflation, and rising food and fuel prices.
About 2.5 million Somalis currently need food aid.
Somalia has not had a functioning national government since 1991 and has been wracked by civil strife.