"We are concerned that pending military operations by the United Arab Emirates and its Yemeni partners will exacerbate the humanitarian crisis by interrupting delivery of humanitarian aid and damaging critical infrastructure", the senators wrote.
"Some civilians are entrapped, others forced from their homes", Jolien Veldwijk-acting country director for the humanitarian group CARE, which is still operating in Yemen-told Reuters on Wednesday as the US-backed Saudi assault on Hodeida began.
Emirati Minister of State for Foreign Affairs Anwar Gargash earlier told French newspaper Le Figaro the deadline for a withdrawal from Hodeida by the Houthis expired early Wednesday morning.
The attack on the Red Sea port aimed to drive out Iranian-aligned Shiite rebels known as Houthis, who have held Hodeida since 2015, and break the civil war's long stalemate.
The coalition says victory could break the stalemate in the war and force the Houthis to the negotiating table.
The United Nations fears the assault could drastically worsen already desperate conditions in the region's poorest country.
The United Nations estimates that 600,000 people live in the area, and in a worst-case scenario, a battle could cost up to 250,000 lives, as well as cutting off aid and other supplies to millions of people.
The U.N.'s humanitarian coordinator for Yemen, Lise Grande, warned that a prolonged siege of the port could lead to the deaths of up to a quarter-million Yemenis due to starvation and disease.
Foreign Secretary Boris Johnson said on Wednesday the British government was in contact with the alliance about ensuring its operations comply with worldwide law on protecting civilians. It has created the world's largest food emergency and led to a cholera outbreak that is thought to have killed 2,290 people.
"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.
A bipartisan group of House lawmakers also sent a letter they had been circulating earlier in the week that asked Mattis to "use all available means to avert a catastrophic military assault".
Defense Secretary Jim Mattis on Monday acknowledged the US continues to provide support to the Saudi-led coalition.
There were even a few scant claims that United States military advisers were on board the coalition warship when it was hit, but Pentagon Spokesman Major Adrian Rankine-Galloway told Business Insider that there are no U.S. military advisers on the ground and he said he hadn't even heard of the ship getting hit. "The administration and the Saudis should take this language as yet another clear signal that Congress will not stand idly by as the US-supported coalition continues to kill and starve civilians in Yemen with impunity".
Since March 2015, Saudi Arabia and some of its Arab allies have been carrying out airstrikes against the Houthi Ansarullah movement in Yemen in an attempt to restore power to fugitive former president Abd Rabbuh Mansour Hadi, a close ally of Riyadh. The Houthis, drawn from a Shi'ite minority that ruled a thousand-year kingdom in Yemen until 1962, deny they are Iranian pawns.
Yemen lies beside the southern mouth of the Red Sea, one of the world's most important trade routes, where oil tankers pass from the Middle East through the Suez Canal to Europe.
Fighting raged mainly near the city's airport and the al-Durayhmi area south of the city, media reports say.