As the US-backed coalition in Yemen appears ready to assault the rebel-held port city of Hodeidah, the entry point for most of the country’s aid, Congress is calling on the Pentagon to “use all available means” to stop the operation.

In a letter circulating in the House of Representatives and obtained by Al-Monitor, members of Congress “deeply alarmed” by the military action are asking US Defense Secretary Jim Mattis to dissuade Saudi Arabia and the United Arab Emirates from attacking. UN officials worry the assault could put as many as 250,000 lives at risk.

The Pentagon has already been scrutinized by lawmakers for refueling Saudi and UAE jets in the yearslong conflict and deploying US special forces troops to dismantle Iran-backed missile launchers near Yemen’s northern border without seeking congressional authorization. Al-Monitor reported in November that US refueling support to the coalition more than doubled over the last year.

“We urge you to use all available means to avert a catastrophic military assault on Yemen’s major port city of Hodeida by the Saudi-led coalition, and to present Congress with immediate clarification regarding the full scope of US military involvement in that conflict,” reads the letter spearheaded by Rep. Mark Pocan, D-Wis., which has been signed by seven members of Congress. “We remind you that three years into the conflict, active US participation in Saudi-led hostilities against Yemen’s Houthis has never been authorized by Congress, in violation of the Constitution.”