The Red Sea is a critical waterway for oil and commodities, but traffic has dropped sharply since Houthi attacks off Yemen's coast began in November 2023 in what the Iran-aligned group said was in solidarity with Palestinians in the Gaza war.

The group sank two ships this week after months of calm and its leader Abdul Malik al-Houthi reiterated there would be no passage for any company transporting goods connected to Israel.

In recent days more ships sailing through the southern Red Sea and the narrow Bab al-Mandab strait have added messages to their AIS public tracking profiles that can be seen when clicking on a vessel.

Messages have included referring to an all-Chinese crew and management, and flagging the presence of armed guards on board.

"All Crew Muslim," read one message, while others made clear the ships had no connection to Israel, according to MarineTraffic and LSEG ship-tracking AIS data.

Maritime security sources said this was a sign of growing desperation to avoid attack by Houthi commandos or deadly drones - but they also thought it was unlikely to make any difference.

Houthi intelligence preparation was "much deeper and forward-leaning", one source said.

Vessels in the broader fleets of both ships attacked and sunk by the Houthis this week had made calls to Israeli ports in the past year, shipping analysis showed.

Maritime security sources said even though shipping companies must step up due diligence on any tangential link to Israel before sailing through the Red Sea, the risk of attack was still high.

In March 2024, the Houthis hit the Chinese-operated tanker Huang Pu with ballistic missiles despite previously saying they would not attack Chinese vessels, the United States Central Command said.

The Houthis have also targeted vessels trading with Russia.

"Despite declared ceasefires, areas such as the Red Sea and Bab al-Mandab Strait remain designated high-risk by underwriters," insurance broker Aon said in a report this week.

"Ongoing monitoring and adaptive security measures are essential for ship operators."

The insurance cost of shipping goods through the Red Sea has more than doubled since this week's attacks, with some underwriters pausing cover for some voyages.

The number of daily sailings through the strait, at the southern tip of the Red Sea and a gateway to the Gulf of Aden, was 35 vessels on July 10, 32 vessels on July 9, down from 43 on July 1, Lloyd’s List Intelligence data showed.

That compares with a daily average of 79 sailings in October 2023, before Houthi attacks began.

"Seafarers are the backbone of global trade, keeping countries supplied with food, fuel and medicine. They should not have to risk their lives to do their job," the UK-based Seafarers' Charity said this week.