Yemeni fighters from the Houthi movement said they will block Saudi Arabia’s coast starting Monday. Their spokesman, Yahya Saree, called it a naval attack. Merchant ships moving through the Red Sea near Saudi ports may encounter holdups. U.S. warplanes persisted in attacking objectives after their warning. The group took several ships. Insurers now see Red Sea routes as hazardous. Traders are watching closely, watching for higher fees or diverted shipments. Saudi ports border the threatened zone, so any slowdown matters fast.