SANAA, YEMEN — The U.S. Navy fired three missiles at Yemen on Thursday, in Washington’s first direct military assault against Iran-backed Houthi forces.

The Pentagon stressed that the strikes were a retaliatory measure, after a U.S. ship in the Red Sea was earlier targeted by missiles from Houthi-controlled territory.

The USS Nitze destroyer fired three Tomahawk missiles at around 4 a.m., Reuters reported. The strikes were aimed at three coastal radar sites, which had enabled the earlier Houthi missile attacks.

The Pentagon was keen to point out that the radar sites were in remote areas and the chance of civilian casualties was low.

The radar sites were used to launch at least three coastal cruise missiles against the USS Mason on Sunday and Wednesday.

The Houthis are fighting the internationally recognized government of Yemen and currently hold the capital city Sanaa.

They’ve denied launching any strikes against U.S. ships and said the allegations were a ruse designed to escalate aggression against the Yemeni people.

The U.S. warned that it would respond to any threat against its Navy or commercial ships in the area.