A bipartisan group of US lawmakers is demanding that President Donald Trump seek the approval of Congress before escalating US involvement in Yemen's civil war.

Four House members are collecting signatures on a letter to the president amid growing signs that the White House and the Pentagon want to more directly take on the Iran-backed Houthi rebels. The United States has been selling bombs and weapons to the Saudi-led coalition since its March 2015 intervention, but the Trump administration is reportedly considering helping Saudi and Emirati forces capture the Red Sea port of Hodeida. 

"Engaging our military against Yemen’s Houthis when no direct threat to the United States exists and without prior congressional authorization would violate the separation of powers clearly delineated in the constitution," reads a draft letter to Trump obtained by Al-Monitor. "For this reason, we write to request that the Office of Legal Counsel (OLC) provide, without delay, any legal justification that it would cite if the administration intends to engage in direct hostilities against Yemen’s Houthis without seeking congressional authorization."

Attorney General Jeff Sessions is copied on the letter.