Some 8.3 million Somalis, almost half the population, require urgent humanitarian assistance, Guterres said, adding that only 15% of the country's $2.6 billion aid requirement for this year has been met.
"When famine looms, this is totally unacceptable," Guterres told reporters in Mogadishu.
He was speaking after visiting a camp in Baidoa for people displaced by the drought and by fighting between al-Shabab, an al-Qaeda affiliate, and government forces.
"It is unconscionable that Somalis, who have done almost nothing to create the climate crisis, are suffering its terrible impact," Guterres said. "Climate change is causing chaos."
After five consecutive failed rainy seasons, the drought has displaced 1.4 million Somalis, with women and children making up 80% of them, he said.
The Integrated Food Security Phase Classification (IPC), which sets the global standard for determining the severity of a food crisis, said last December that famine had been temporarily averted but warned the situation was getting worse.
A major government offensive backed by allied clan militias has captured around a third of al-Shabab's territory, the U.S. ambassador to Somalia told Voice of America in March.
The army and its clan allies are expected to start a second phase of the operation, backed by the African Union Transition Mission in Somalia (ATMIS), in the coming weeks.