The Huthi rebels have attacked civilian and military ships sailing off Yemen's shores at least 50 times since their assaults began late last year, a senior US Defense Department official said Thursday.

The Iran-backed Huthis have been striking merchant vessels transiting the vital Red Sea trade route for months despite repeated US and British air strikes against them.

"In the Red Sea, the Huthis seek to affect this vital channel for global trade with at least 50 attacks against commercial shipping and naval vessels," Assistant Secretary of Defense Celeste Wallander told lawmakers.

The Huthis began attacking ships in the Gulf of Aden and the Red Sea in November, a campaign they say is intended to signal solidarity with Palestinians in Gaza.

They have vowed to strike Israeli, British and American ships, as well as vessels heading to Israeli ports, disrupting traffic through the vital trade route off Yemen's coasts.

The Huthi attacks have sent insurance costs spiraling for vessels plying the key Red Sea trade route and prompted many shipping firms to take the far longer passage around the southern tip of Africa instead.