But many ships are still plying the waterway. Several ships underway have armed guards on board, LSEG data showed.

At least 11 container ships which had passed through Suez and were approaching Yemen carrying consumer goods and grains bound for countries including Singapore, Malaysia and the United Arab Emirates, are now anchored in the Red Sea between Sudan and Saudi Arabia, LSEG shiptracking data showed.

Four MSC container ships in the Red Sea have had their transponders turned off since Dec. 17, the data showed, likely to avoid detection.

Three liquefied natural gas (LNG) vessels have also adjusted their routes to avoid passing by Yemen, according to shiptracking data by Kpler and LSEG Eikon.

Vessels are attempting to mask their positions by pinging on other locations, as a safety precaution when entering the Yemen coastline, said Ioannis Papadimitriou, senior freight analyst at Vortexa.

“Most ships will be turning off their AIS (transponders) at some point in those waters,” one shipping industry source said.

Analysis from maritime AI company Windward showed the number of incidents involving cargo vessels going dark within waters around Saudi Arabia’s exclusive economic zone in the Red Sea rose to 81 in November from 74 in October, after averaging 42 incidents between October 2022 to September 2023.

A Dec. 15 advisory issued by leading shipping associations said ships that switched AIS on and off had been attacked.

“Switching off AIS makes it marginally more difficult to track a ship, but may also hinder the ability of the military to provide support or direct contact,” the advisory said.