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President Joe Biden said Thursday that US and British forces had launched air strikes against Iran-backed Houthi rebels in Yemen in "defensive action" after attacks on shipping in the Red Sea.

In a statement, Biden said he "will not hesitate" to order further military action if needed.

Air strikes hit a number of cities in Yemen, where the Houthis control a swathe of territory, a Houthi source and witnesses said.

The strikes involved fighter jets and Tomahawk missiles, several US media said.Ìý

"Today, at my direction, US military forces -- together with the United Kingdom and with support from Australia, Bahrain, Canada, and the Netherlands -- successfully conducted strikes against a number of targets in Yemen used by Houthi rebels to endanger freedom of navigation in one of the world's most vital waterways," Biden said, using an alternate spelling of Houthi.

He called the strikes a "direct response" to "unprecedented" attacks by the Houthis, "including the use of anti-ship ballistic missiles for the first time in history."

"These attacks have endangered US personnel, civilian mariners, and our partners, jeopardized trade, and threatened freedom of navigation," he said, stressing that Washington and its allies "will not tolerate attacks on our personnel or allow hostile actors to imperil freedom of navigation."

"I will not hesitate to direct further measures to protect our people and the free flow of international commerce as necessary."

The Houthis have carried out a growing number of attacks on the key international sea route in retaliation to Israel's indiscriminate war on Gaza, which both the US and UK support.Ìý

In his statement, Biden said that on January 9 the Houthis "launched their largest attack to date -- directly targeting American ships."

But the Western strikes risk turning an already tense situation in the Middle East into a wider conflagration pitting the United States and Israel against Iran and its regional proxies.

The Houthi rebels say they are acting in response to Israel's bombardment of the Gaza Strip, and have launched a series of drones and missiles towards Israel.

They have controlled a major part of Yemen since a civil war erupted there in 2014, and are part of the Iran-backed so-called "axis of resistance" arrayed against Israel.

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The United States and its allies had issued a series of increasingly stern warnings to the H0uthis to stop the shipping attacks, although Washington has been cautious about the risks of further inflaming regional tensions.

Washington set up an international coalition in December -- dubbed Operation Prosperity Guardian -- to protect maritime traffic in the area, through which 12 percent of world trade flows.

Twelve nations led by the United States then warned the Houthis on January 3 of "consequences" unless they immediately stopped attacks on commercial vessels.

But late Tuesday the Houthis launched what London called the most significant attack yet by the Yemeni rebels, with US and British forces shooting down 18 drones and three missiles.

That prompted warnings from Britain, while the UN Security Council also Wednesday urged an immediate halt to the attacks.

The final straw for the Western allies appeared to come early Thursday when the US military said the Houthis fired an anti-ship ballistic missile into a shipping lane in the Gulf of Aden.

It was the 27th attack on international shipping in the Red Sea since November 19, the US military said.

The intensifying attacks have caused shipping companies to divert around South Africa's Cape of Good Hope, sparking fears of a shock to the global economy.

The Houthis say they only target vessels linked to Israel or its allies.

The United States strengthened its military posture in the region right after Israel began its war on Gaza, and warned Iran and its allies not to escalate the situation.

The Biden administration was initially cautious in its response as it is seeking to preserve a fragile peace in Yemen, where a decade of civil war and a Saudi-led coalition's military campaign has caused one of the world's worst humanitarian crises in the Arabian peninsula's poorest country.