Spiraling attacks on merchant ships by Houthi militants off the coast of Yemen have prompted widespread trade disruption with some of the world’s biggest vessel owners evaluating whether it’s safe to send crews through the Red Sea.

Two of the world’s largest container shipping lines said on Friday that they were pausing transits through the Red Sea after their vessels were attacked. Two oil tanker companies have now said they are insisting on a clause in charters that will allow them to send their ships around Africa if they deem the waters off Yemen unsafe.