Here are four things to know about the rebels:

- Iranian support -

The Huthis are supported by Iran, a major regional power and sworn enemy of Israel.

They are members of the Tehran-backed "axis of resistance", which brings together various anti-Israel organisations in the region, including Palestinian Hamas militants and Lebanon's Hezbollah, as well as diverse groups in Iraq and Syria.

Since the start of the Israel-Hamas war on October 7, the Huthis have launched numerous attacks against commercial shipping they say is linked to Israel, in solidarity with Palestinians in the Gaza Strip.

The United States, which has deployed warships to the Red Sea and brought together an international coalition to secure the internationally vital shipping lane, has accused Iran of supporting and encouraging the attacks. Tehran denies the accusation.

- Military strength -

The group's military strength was estimated several years ago at around 200,000 fighters, a force which is well-trained and accustomed to combat in Yemen's arid and mountainous terrain.

After taking control of the capital Sanaa in 2014, the Huthis seized vast swathes of the country, in particular in the north.

Their long-distance missiles and drones, developed using Iranian technology, according to their opponents, are considered a serious menace by neighbouring countries in the Gulf.

The Huthis have previously carried out attacks against Saudi Arabia and the United Arab Emirates, both members of a coalition that supported the Yemeni government against the Huthis since 2015.

However, until now, they had not intervened in conflicts not involving Yemen.

Having fought against the Huthis for more than eight years, Riyadh began peace talks with the group last year in the hope of bringing to an end a conflict that has devastated one of the world's poorest countries.

Fighting has largely been on hold since a truce in early 2022.

- Popularity -

Despite thousands of its fighters being killed, the Huthis continue to attract young recruits in a country of about 30 million people struggling with one of the world's worst humanitarian disasters.

The civil war has killed hundreds of thousands of people directly or indirectly and left millions on the edge of famine, according to the United Nations.

By claiming to carry out attacks on vessels in the Red Sea linked to Israel and the US in solidarity with the Palestinians in Gaza, the Huthis have gained global visibility and rallied their popular base both in Yemen and across the region, experts say.

- Shiite minority origins -

Originating in the north of Yemen, the Huthis were founded as a movement in the 1990s to fight the alleged marginalisation of their minority Zaidi community, a branch of Shiite Islam, in the majority Sunni country.

The Zaidis reached their apogee in the north of Yemen with the creation of an "imamate" (a political regime run by an imam) in the 9th century, which survived until the mid-20th century.

In the territory under their control, the Huthis have imposed an extremely strict social and religious rules which target women in particular.