The Iran-backed Huthi rebels in Yemen warned allies of Israel Wednesday that their shipping in the Bab al-Mandab Strait at the entrance to the Red Sea is a "legitimate target".

The warning comes after the Huthis on Sunday seized an Israel-linked cargo vessel, opening a new dimension in the Gaza war between Israel and the Palestinian Islamist group Hamas.

Helicopter-borne troops seized the Galaxy Leader and its 25 international crew days after the Huthis threatened to target Israeli shipping because of the war.

The ship had been chartered by a Japanese group, and was flying the flag of the Bahamas.

A photographer who collaborates with AFP said the Galaxy Leader was anchored at the rebel-controlled Hodeida port in northwest Yemen and was closely guarded.

It was now flying the Yemeni and Palestinian flags.

The Huthis have declared themselves part of the "axis of resistance" of Iran's allies and proxies, and have also launched a series of drones and missiles towards Israel.

A video posted on X, formerly Twitter, by Huthi military media showed the commander of the rebels' naval forces, General Mohammad Fadl Abdelnabi, aboard the captured vessel.

"Allies of the Zionist enemy who ensure passage through Bab al-Mandab are also considered to be a legitimate target," he said of the choke point at the foot of the commercially vital Red Sea.

"We say to the Zionist entity that Bab al-Mandab is a red line… Every civilian or military (Israel-affiliated) ship is considered a legitimate target," he added.

The vital seaway is a narrow strait between Yemen and Djibouti through which a significant portion of the world's maritime traffic passes.

One of the world's busiest shipping lanes, it carries about a fifth of global oil consumption.

Specialist website Marine Traffic said the Galaxy Leader, a car transporter, was off the Saudi port of Jeddah on Saturday when its radar signature disappeared.

Israel's military on Sunday said the seizure was a "very grave incident of global consequence", and a US military official said it was "a flagrant violation of international law".

Israel said the ship had sailed from Turkey bound for India, and that there were no Israelis on board.

On Monday the Huthis released a video purporting to show Sunday's seizure.

The footage showed masked armed men jumping onto the ship from a helicopter while the vessel was still moving, and holding crew members at gunpoint.

"We will undertake combat missions until the Zionist entity stops attacking Gaza," the Huthi general said in the latest video footage on X.