The Iran-aligned Houthis of Yemen are playing an escalating role in the conflict in the Middle East, attacking shipping in the Red Sea and firing drones and missiles at Israel in a campaign they say aims to support Palestinians in the Gaza war.

The U.S. envoy for Yemen told Reuters Washington wants to form the "broadest possible" maritime coalition to protect ships in the Red Sea and send an "important signal" to the Houthis that further attacks will not be tolerated.

The Houthis' role has added to the conflict's regional risks, threatening sea lanes through which much of the world's oil is shipped, and worrying states on the Red Sea as Houthi rockets and drones fly towards Israel.

Who are the Houthis?

HISTORY

In the late 1990s, the Houthi family in far north Yemen set up a religious revival movement for the Zaydi sect of Shi'ite Islam, which had once ruled Yemen but whose northern heartland had became impoverished and marginalised.

As friction with the government grew, they fought a series of guerrilla wars with the national army and a brief border conflict with Sunni powerhouse Saudi Arabia.

GROWING POWER

Their power grew during the Yemen war which began in late 2014, when they seized Sanaa. Worried by the growing influence of Shi'ite Iran along its border, Saudi Arabia intervened at the head of a Western-backed coalition in 2015 in support of the Yemeni government.

The Houthis established control over much of the north and other big population centres, while the internationally recognised government based itself in Aden.

Yemen has enjoyed more than a year of relative calm amid a U.N.-led peace push. Saudi Arabia has been holding talks with the Houthis in a bid to exit the war.

ROLE IN MIDEAST WAR

The Houthis waded into the latest conflict as it spread around the Middle East, announcing on Oct. 31 they had fired drones and missiles at Israel and vowing they would continue to mount attacks "until the Israeli aggression stops".

Their actions have echoed the role of the Iran-backed Lebanese group Hezbollah, which has been attacking Israeli positions at the Lebanese frontier, and Iraqi militias which have been firing at U.S. interests in Iraq and Syria.

Stepping up their threats, the Houthis said on Dec. 9 they would target all ships heading to Israel, regardless of nationality, and warned all international shipping companies against dealing with Israeli ports.

In the latest incident, the U.S. military said it shot down an aerial vehicle launched from a Houthi-controlled area in response to a call from a Marshall Islands-flagged tanker.

The Houthis' slogan is "Death to America, Death to Israel, curse the Jews and victory to Islam".

IRAN LINKS

The United States believes that Iran's Islamic Revolutionary Guard Corps is helping to plan and carry out the Houthi missile and drone attacks.

Iran denies involvement.

The Saudi-led coalition has long accused Iran of arming, training and funding the Houthis. The Houthis deny being an Iranian proxy and say they develop their own weapons.

ARSENAL

The Houthis demonstrated their missile and drone capabilities during the Yemen war in attacks on Saudi Arabia and the United Arab Emirates, targeting oil installations and vital infrastructure.

The arsenal includes ballistic missiles and armed drones capable of hitting Israel more than 1,000 miles from their seat of power in Sanaa.

Its Tofan, Borkan, and Quds missiles are modelled on Iranian weapons and can hit targets up to 2,000 km (1,200 miles) away, experts say.

The Houthis fired these missiles at Saudi Arabia dozens of times during the Yemen war. In September, the Houthis displayed anti-aircraft Barq-2 missiles, naval missiles, a Mig-29 fighter jet and helicopters for the first time.

The Houthis have also used fast boats armed with machine guns in their operations against shipping.