Guterres "is gravely concerned about the air strikes conducted by the United States over the course of 17 and 18 April in and around Yemen's port of Ras Issa, which reportedly resulted in scores of civilian casualties, including five humanitarian workers injured," spokesman Stephane Dujarric said in a statement.

The strikes on Ras Issa aimed to cut off supplies and funds for the Huthi rebels that control large swathes of the Arabian Peninsula's poorest country, the US military said.

It was the deadliest attack of Washington's 15-month campaign against the Iran-backed group.

Guterres expressed fears of damage to the port and "possible oil leaks into the Red Sea," Dujarric said.

He also called on the Huthis to halt their ongoing missile and drone attacks on both Israel and shipping in the Red Sea "immediately."

The US military has hammered the Huthis with near-daily air strikes for the past month in a bid to finally stamp out their attacks on shipping in the Red Sea and Gulf of Aden.

The Huthis say their attacks are in protest at Israel's blockade of aid to Gaza, where it is waging a bloody war on Palestinian militant group Hamas.

Dujarric said Guterres remained "deeply troubled" by the threat of further escalation in the region, and that he urged all sides to "exercise utmost restraint."