Human Rights Watch on Wednesday condemned Yemen's Houthi group for deadly attacks that sank two commercial vessels this month, calling them violations of the laws of war.

The Houthis struck the Magic Seas and Eternity C cargo ships in the Red Sea, part of a campaign against maritime traffic they accuse of having links to Israel, launched over the Gaza war.

Fifteen people -- including four confirmed dead -- remain missing after the July 7 attack on the Eternity C.

The Yemeni rebels claimed to have "rescued" an unspecified number of crew, whose whereabouts are still unknown.

The attacks were "violations of the laws of war amounting to war crimes", Human Rights Watch said in a statement, adding it found "no evidence that the ships were military targets".

"They deliberately attacked commercial vessels that could clearly be identified as civilian," the New York-based group said, adding that "detaining rescued crew members is also prohibited".

Rebel leader Abdel Malek al-Houthi justified the attacks, saying both ships belonged to companies serving Israeli ports.

But HRW said the ships had no connection to Israel and were not heading there.

The Magic Seas was en route to Turkey from China carrying fertiliser and steel billets when it was attacked on July 6.

The Eternity C was heading to Saudi Arabia from Somalia after delivering humanitarian aid for the United Nations World Food Programme.

"The Houthis have sought to justify unlawful attacks by pointing to Israeli violations against Palestinians," said Niku Jafarnia, HRW's Yemen and Bahrain researcher.

"The Houthis should end all attacks on ships not taking part in the conflict and immediately release the crew members in their custody," she added.

Since November 2023, the rebels have carried out more than 100 attacks on vessels in the Red Sea and Gulf of Aden, according to the Joint Maritime Information Centre, run by a Western naval coalition.

HRW said it had previously found those actions to be war crimes.

It also warned of environmental risks, citing findings by Wim Zwijnenburg of Dutch peace organisation PAX.

Zwijnenburg said satellite imagery showed large oil slicks trailing from the sites where both vessels sank, threatening wildlife in a protected nature reserve off Eritrea's coast.

Oil was also reportedly washing ashore near a fishing community, he was quoted as saying.