Fighting between the United States and Iran-backed Houthi militants appeared to intensify on Sunday, after the U.S. military killed more than 50 people in Yemen with airstrikes a day prior.

Dozens of people have been reported killed after US President Donald Trump ordered “decisive” military action against Houthi rebels in Yemen, opening a new salvo against the Iran-backed group that has targeted shipping lanes in the Red Sea.

The strikes represent the biggest U.S. military operation in the Middle East since Trump took office, and come as the U.S. increases sanctions pressure on Iran while trying to bring it to the negotiating table over its nuclear program.