Assault on Yemen's largest port threatens to increase mass starvation As many as 22 million people could be at risk.

SEOUL, South Korea -- A military coalition led by Saudi Arabian and United Arab Emirates forces, with vital guidance and supplies provided by the United States, has launched the largest assault of Yemen's war, with an attack on the massive port city of Hodeidah.

As many as 22 million people – three-quarters of Yemen’s population – could be at risk of losing access to necessary food and medicines they receive through the port, amid a worsening humanitarian crisis on the verge of famine that the U.N. has described as the world’s most dire.

"Any attack on or significant, long-term disruption of operations of the port will have catastrophic consequences for the people of Yemen," Frank McManus, the International Rescue Committee's country director in Yemen, told ABC News.

Saudi Arabian warplanes and warships began pounding the fortifications of the Iranian-supplied Houthi rebels this morning, to support ground operation “Golden Victory” led by Yemeni troops massed south of the port of Hodeidah, witnesses told Reuters.

The assault threatens the safety of humanitarian workers in the city, as well as around 400,000 people living there, McManus said.

"We condemn to the fullest extent the launch of an attack by the Saudi and Emirati-led coalition on Yemen's port city of Hodeidah," he said.

Human rights groups say airstrikes carried out by the Saudi-led coalition have killed and wounded thousands of civilians, often in indiscriminate attacks.

Wednesday's military operations mark the first time the Arab states have launched such a wide scale attack since joining the war three years ago against the Iran-backed Houthi rebels, who control the capital Sanaa and most of the populated areas of northern Yemen.

The U.S. has backed the Saudi Arabian and Emirati forces through diplomacy, selling them billions of dollars per year in arms and providing logistical support such as warplane refueling and military intelligence.

A major battle that throws the people of Yemen into further crisis and starvation could test that support.

"Hodeidah is absolutely essential to the preserving of life," the United Nation's emergency relief coordinator, Mark Lowcock, said this week. "If for any periods Hodeidah were not to operate effectively, the consequences, in humanitarian terms, would be catastrophic."

Lowcock briefed the U.N. Security Council in a closed-door session on Monday and spoke to reporters afterwards.

"Ninety percent of the food and fuel and the medicines that are consumed in Yemen are imported, 70 percent of them come through Hodeidah,” he said. "Seven million people are completely relying every month on food and more than seven million on other assistance, from humanitarian organizations."

Wednesday, the U.N. further warned that escalating violence in Yemen could lead to a refugee exodus as more people flee the famine and fighting.

"It's actually surprising that an exodus has not happened yet," United Nations refugee chief Filippo Grandi told reporters in Geneva.