As American warships rack up kills against Houthi drones and missiles in the Red Sea, Pentagon officials are increasingly alarmed not just at the threat to U.S. naval forces and international shipping — but at the growing cost of keeping them safe.

U.S. Navy destroyers have shot down 38 drones and multiple missiles in the Red Sea over the past two months, according to a Defense Department official, as the Iran-backed militants have stepped up attacks on commercial vessels moving energy and oil through the world’s most vital shipping lanes. On Saturday alone, the destroyer USS Carney intercepted 14 one-way attack drones.

Houthi leaders have said the attacks are a show of support for the Palestinians, and that they won’t stop until Israel halts its operations in Gaza. Defense Secretary Lloyd Austin on Monday announced a new international maritime coalition to safeguard shipping and counter the attacks.

The cost of using expensive naval missiles — which can run up to $2.1 million a shot — to destroy unsophisticated Houthi drones — estimated at a few thousand dollars each — is a growing concern, according to three other DOD officials. The officials, like others interviewed for this story, were granted anonymity to describe sensitive operations and internal deliberations.