The letter comes on the heels of another, signed by 55 legislators, to President Trump and Attorney General Jeff Sessions insisting that any direct U.S. involvement in Yemen be brought before Congress for authorization. In a trip to Saudi Arabia in April, Mattis hinted at direct U.S. military and intelligence support for the Saudi-led coalition, which is seeking to dislodge the Shiite-led Houthi rebels from Sanaa and other areas they control in Yemen.

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Saudi fighter jets dropped leaflets over Houthi-controlled Hodeida in recent days warning its hundreds of thousands of residents of an impending offensive, according to the United Nations and aid agencies. Yemen imports 90 percent of its food, and Hodeida's already-damaged port is the entry point for the vast majority of it. A two-year-long civil war has destroyed Yemen's economy, and more than 7 million people rely on humanitarian aid for survival.

Human rights activists have accused the Saudis of indiscriminate bombing in its campaign, saying it has killed thousands of civilians and reduced much of Yemen's vital infrastructure to rubble.

Mattis has asked for the scrapping of Obama-era rules prohibiting direct support to the coalition. Although the Obama administration sold weapons and refueled aircraft to the coalition, direct U.S. engagement through special teams was ruled out because it was considered ineffective in thwarting the well-armed Houthis and because of humanitarian concerns.

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But the Trump administration has taken a harder line on the Houthis, driven by Saudi allegations that Iran funds, trains and arms the rebels as part of a proxy war against the Sunni monarchies that make up the bulk of the coalition. The Saudis allege that weapons are smuggled through Hodeida to the Houthis, providing the rebels a vital lifeline.

On Monday, U.N. human rights chief Zeid Ra'ad al Hussein echoed the exhortations of aid organizations, saying, “The U.N. is concerned about the humanitarian repercussions of such an attack in terms of inflaming the humanitarian crisis even further, let alone our concerns about loss of civilian life were there to be a large-scale attack on port.”

Tuesday's letter, drafted by Mark Pocan (D-Wis.) and Justin Amash (R-Mich.), asserts that any direct U.S. support for an offensive on the Houthis must be authorized by Congress. The letter also threatens legislation that would seek to “prohibit U.S. involvement in any such assault” should Mattis fail to brief Congress on the nature of U.S. support for the coalition.

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“Last month, in a bipartisan request, 54 of my colleagues and I asked President Trump a simple question: what legal justification is the White House claiming for escalating U.S. involvement in Saudi Arabia’s war in Yemen — a war that’s never been authorized by Congress?” Pocan said in an email. “With a potential green light from President Trump, the Saudis appear to be gearing up to destroy the lifeline to food imports for millions of Yemenis on the verge of starvation.”

Since the Houthis took control of the Hodeida port, the Saudi navy has imposed a de facto blockade, allowing only a trickle of ships to dock. Saudi jets have also damaged many of the port's cranes, making the unloading process difficult and time-consuming. The Saudis rarely take part in ground operations, and an assault on Hodeida would probably involve a large number of troops from the United Arab Emirates, a coalition partner.

Since Hodeida is densely populated, and the port is surrounded by the bustling city, an assault could take weeks, if not months, and lead to a mass exodus of residents as well as the tens of thousands of internally displaced people sheltering there.

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“The big question is how do they take the city without destroying it and the port in the process,” said Scott Paul, a senior humanitarian policy adviser at Oxfam International who has worked in Yemen. “With any closure, we'd almost certainly have a famine in just a few months.”

Administration officials have expressed concern about the humanitarian fallout of such an assault but have cautioned that U.S. interests in the region are increasingly at risk unless Washington acts.

Houthi naval aggression has made traversing the narrow Bab el-Mandeb Strait, where thousands of ships bound to and from the Suez Canal travel, hazardous.

In October, Houthis attacked U.S. and allied ships crossing the Red Sea, prompting the United States to use Tomahawk missiles to destroy Houthi coastal radar sites. Officials said a Houthi missile aimed at a U.S. naval vessel was fired from Hodeida.

The loss of Hodeida could force the Houthis back to the negotiating table, as the coalition hopes, but the bloodshed expected could also deepen the war's divisions and prolong Yemen's immense suffering.