Yemen’s Houthi movement attacked two commercial ships in the Red Sea with naval drones on Monday, the Iran-backed group said, the latest in a surge of attacks over Israel’s military campaign in Gaza.

Houthi spokesperson Yahya Sarea identified the vessels as the MSC Clara and Norwegian-owned Swan Atlantic, and said the attacks were carried out after their crews failed to respond to calls from the group.

The Swan Atlantic’s owner said the ship had been struck by an unidentified object but none of the crew was hurt.

The MSC Clara is a Panama-flagged vessel, according to LSEG data. Details of the attack on the vessel were not immediately known.

The Houthis say they have been attacking vessels in the Red Sea with links to Israel in protest at its military offensive against Hamas in Gaza, and have warned against sailing towards the area.

The attacks have caused concerns about the impact on the passage of oil, grain and other goods on what is an important global trade route, and have pushed up the cost of insuring and shipping goods through the Red Sea.

Soon after the latest attacks, London’s insurance market widened the area in the Red Sea it deems as high risk.

Taiwan’s Yang Ming Marine Transport Corporation said it would divert any of its ships sailing through the Red Sea or Gulf of Aden via the Cape of Good Hope for the next two weeks.

Norway-based oil tanker group Frontline said its vessels would also start avoiding passages through the Red Sea and the Gulf of Aden.

“War risk insurance premiums are on the rise naturally, but as vessels gets re-routed around Africa shipping supply will be tighter as cargoes travel longer. That would put rates under a strong upwards pressure,” CEO Lars Barstad told Reuters.

The announcement followed decisions by two major freight firms including MSC, the world’s biggest container shipping line, to avoid the Suez Canal in response to the attacks by Houthi forces, which control most of Yemen, a poor Arab state.

The Suez Canal shipping route, which leads to the Red Sea, is a vital waterway for global trade, used to transport energy and other goods between Europe and Asia, and elsewhere. The route saves on time and expense by avoiding having to navigate around the entire Africa continent.