Six people were killed Tuesday when Iran-backed Houthi rebels fired missiles at a vessel in Yemen’s Bab el-Mandeb strait, including four crew members and two allied fighters.

The attack marks the first known deaths from Houthi attacks on shipping off Yemen during the latest escalation of fighting.

The violence comes as the Houthis have increased attacks on Yemeni government forces, Saudi Arabia and shipping in the Red Sea.

Officials warn the attacks could reignite Yemen’s civil war and create another front in the broader Middle East conflict, the AP has reported.

Houthi officials said they remain open to negotiations but demanded an end to the Saudi-led air and sea blockade of Yemen.