US and Israeli forces launched a massive air campaign against Iran on February 28. Iran has retaliated with strikes against Israel and Gulf countries.

Washington said it aimed to curb nuclear and missile threats from Tehran but it has also decapitated the country's government, and President Donald Trump is now demanding "unconditional surrender."

"All the unlawful attacks in the Middle East and beyond are causing tremendous suffering and harm to civilians throughout the region -- and pose a grave risk to the global economy, particularly to the most vulnerable people," Guterres said.

"The situation could spiral beyond anyone's control. It is time to stop the fighting and get to serious diplomatic negotiations."

The UN's humanitarian chief Tom Fletcher said Friday that "we're seeing staggering amounts of money, reportedly a billion dollars a day, funding this war spent on destruction, while politicians continue to boast about cutting aid budgets for those in greatest need."

"We're seeing an increasingly deadly alliance of technology and killing with impunity," he added.

Fletcher said "we're seeing a sustained attack against the systems and laws meant to restrain us from our worst instincts and from reckless warfare."

He also reflected mounting concern about the war's other impacts, warning that "it tears through markets, supply chains, food prices, and when that happens, it's the most vulnerable people who are hit first and hardest."