A military coalition led by Saudi Arabia and the United Arab Emirates (UAE) launched an offensive Wednesday on a key port in Yemen held by rebels, despite international warnings such an assault could be catastrophic.

In the United States, which has supported the coalition, lawmakers in both parties issued stern warnings ahead of the offensive on Hodeida.

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A bipartisan quartet of senators, including Senate Foreign Relations Committee Chairman Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn) and ranking member Bob Menendez Robert (Bob) MenendezKasie Hunt to host lead-in show for MSNBC's 'Morning Joe' Senators ask for removal of tariffs on EU food, wine, spirits: report VOA visa decision could hobble Venezuela coverage MORE (D-N.J.), sent a letter to Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE and Secretary of State Mike Pompeo Michael (Mike) Richard PompeoTreasury sanctions individuals, groups tied to Russian malign influence activities Navalny released from hospital after suspected poisoning Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers MORE on the eve of the offensive expressing “grave alarm.”

“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. “We are also deeply concerned that these operations jeopardize prospects for a near-term political resolution to the conflict.”

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.” The letter garnered 34 signatures, according to the copy publicly released Wednesday.

The Trump administration tried to dissuade the coalition from launching the offensive, with Pompeo saying in a statement Monday that he “made clear our desire to address their security concerns while preserving the free flow of humanitarian aid and life-saving commercial imports.”

Having failed to stop the UAE from acting, the U.S. military has helped Gulf allies develop a list of targets that should be off-limits, according to several reports citing unnamed U.S. officials.

Hodeida is controlled by Houthi rebels who are fighting the Saudi-led coalition, and the coalition believes the port has been key to the rebels smuggling in arms.

But humanitarian groups and experts have warned that an offensive on Hodeida, through which 80 percent of Yemen’s aid comes, could devastate the already war-ravaged country. The United Nations said Friday the worst-case scenario is 250,000 civilians killed in the assault.

Yemen’s exiled government said in a statement that it “has exhausted all peaceful and political means to remove the Houthi militia from the port of Hodeida,” according to The Associated Press. “Liberation of the port of Hodeida is a milestone in our struggle to regain Yemen from the militias.”