The United States and Britain launched strikes from the air and sea against Houthi military targets in Yemen in response to the movement’s attacks on ships in the Red Sea, a dramatic regional widening of the Israel-Hamas war in Gaza.

As witnesses in Yemen confirmed explosions throughout the country to Reuters, President Joe Biden cautioned in a statement late on Thursday he would not hesitate to take further action if needed.

“These targeted strikes are a clear message that the United States and our partners will not tolerate attacks on our personnel or allow hostile actors to imperil freedom of navigation,” Biden said.

Britain’s Ministry of Defence said in a statement that “early indications are that the Houthis’ ability to threaten merchant shipping has taken a blow.”

The Iran-backed Houthis say their attacks on shipping routes in the Red Sea are a show of support for the Palestinians and Hamas, the group that controls Gaza.

U.S. Defense Secretary Lloyd Austin, who is in hospital due to surgery complications, said in a statement that the strikes targeted Houthi capabilities including drones, ballistic and cruise missiles, coastal radar and air surveillance.

A Houthi official confirmed “raids” in the capital Sanaa along with the cities of Saada and Dhamar as well as in the Hodeidah governorate, calling them “American-Zionist-British aggression.”

Witnesses told Reuters that the raids targeted a military base adjacent to Sanaa airport, a military site near Taiz airport, a Houthi naval base in Hodeidah and military sites in Hajjah governorate.

Fears of Escalation

The U.S. said Australia, Bahrain, Canada and the Netherlands supported the operation and sought to present the strikes as part of an international effort to restore the free flow of trade in a key route between Europe and Asia that accounts for about 15% of the world’s shipping traffic.

But the strikes, the first on Yemeni territory since 2016, were also an undeniable demonstration of Washington’s struggle to contain the fallout Israel-Hamas war in the Middle East since its eruption in October. Although Washington said there was no intent to escalate tensions, the Houthis have vowed to retaliate to any attack.

“The concern is that this could escalate,” said Andreas Krieg at King’s College in London, warning of the risk that Saudi Arabia and the United Arab Emirates could also be drawn into the confrontation.

In a statement after the strikes, Saudi Arabia called for restraint and “avoiding escalation”.

The United States also accused Iran of being involved operationally in the Houthi attacks on the Red Sea, providing the military capabilities and intelligence to carry them out.

“We believe that they have been certainly involved in every phase of this,” a senior U.S. official told reporters.

The strikes were carried out by aircraft, ships and submarines. A different U.S. official said more than a dozen locations were targeted and the strikes were intended to weaken the Houthis’ military capabilities, as opposed to being just symbolic.

“We were going after very specific capability in very specific locations with precision munitions,” a U.S. military official said.

The Houthis, who control most of Yemen, defied a U.N. and other international calls to halt their missile and drone attacks on Red Sea shipping routes and warnings from the United States of consequences if they failed to do so.

The Houthi attacks have disrupted international commerce, forcing international shipping to take the long route around South Africa to avoid being struck. The increase in delivery costs is stoking fears it could trigger a fresh bout of global inflation.

Israel has launched a military assault that has killed more than 23,000 Palestinians in Gaza after Hamas attacked Israel on Oct. 7, in which 1,200 people died and 240 were abducted.

ANTI-SHIP BALLISTIC MISSILES Hours before the U.S. and British strikes in Yemen, the U.S. military said the Houthis fired an anti-ship ballistic missile into international shipping lanes in the Gulf of Aden, two days after the Houthis’ largest attack to date.

On Jan. 9, U.S. and British naval forces shot down 21 Houthi drones and missiles fired in what the U.S. military described as a complex attack, language reminiscent of the way it once described Taliban or ISIS attacks in Afghanistan and Iraq.

Biden, in his statement, said the Houthi’s Jan. 9 attack directly targeted American ships.

Prominent Republicans in Congress welcomed the move, while some of Biden’s Democrats expressed concern the U.S. could get entangled in another decades-long war.

“This strike was two months overdue, but it is a good first step toward restoring deterrence in the Red Sea,” U.S. Senator Roger Wicker, the top Republican on the Senate Armed Services Committee, said in a statement.

The strikes followed other diplomatic and military steps that U.S. officials had hoped would halt the Houthi attacks without starting a fresh conflict.

In December, more than 20 countries agreed to participate in a defensive U.S.-led coalition, known as Operation Prosperity Guardian, safeguarding commercial traffic in the Red Sea. But the U.S. and British strikes are taking place outside that coalition.

“The response of the international community to these reckless attacks has been united and resolute,” Biden said.