The U.S. military has unleashed large-scale retaliatory strikes against multiple Houthi targets in Yemen following months of attacks by the Iranian-backed militants on commercial shipping in the Red Sea, a U.S. official said Thursday.

The official said that the strikes involve a mix of fighter jets and Tomahawk cruise missiles launched from Navy surface ships and a U.S. Navy submarine, according to two U.S. officials.

One of the officials identified the submarine as the Ohio-class ballistic missile submarine USS Florida, which was seen entering the Red Sea via the Suez Canal on Nov. 5, a move publicized by U.S. Central Command.

Another U.S. official confirmed that the United Kingdom also was using its military assets to launch strikes against Houthi targets.

President Joe Biden, who had been under pressure to respond, issued a statement calling the move a “defensive action” after extensive warnings.

“Today, at my direction, U.S. 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,” he said.

“These strikes are in direct response to unprecedented Houthi attacks against international maritime vessels in the Red Sea — including the use of anti-ship ballistic missiles for the first time in history. These attacks have endangered U.S. personnel, civilian mariners, and our partners, jeopardized trade, and threatened freedom of navigation,” his statement said.

The strikes come after the Iranian-backed militants have repeatedly used drones and missiles to target commercial ships in the Red Sea and the Gulf of Aden, supposedly in support of Hamas in its war with Israel.

On Thursday, the Pentagon disclosed that a Houthi missile had landed harmlessly in the Gulf of Aden after targeting a commercial vessel marking the 27th such attack since NoV. 19.

Last week, the U.S. and other nations released a joint statement warning that “the Houthis will bear the responsibility for the consequences should they continue to threaten lives, the global economy, or the free flow of commerce in the region’s critical waterways.”

But on Tuesday the Houthis ignored the warning by launching their biggest barrage yet as American and British destroyers, along with U.S. Navy jets, shot down 21 drones and missiles aimed at dozens of ships in the Red Sea.

Since then, senior U.S. officials had issued new warnings against the Houthis to stop with the attacks.

“The Houthis need to stop these attacks,” John Kirby, coordinator for strategic communications at the National Security Council, told White House reporters on Thursday. “They will bear the consequences for any failure to do so.”

However, Kirby said the U.S. was “not going to telegraph our punches one way or another here.”

In London, British Prime Minister Rishi Sunak convened a meeting of his top national security officials and members of Parliament were briefed leading to speculation that retaliatory strikes were imminent.

The United Kingdom forms part of the 20 countries that make up the U.S.-led Operation Prosperity Guardian established in late December to defend commercial vessels from Houthi attacks as they transited the Red Sea.

The shipping route through the Bab-el-Mandeb Strait into the Red Sea is a vital waterway through which 15% of the world’s commerce transits.

The attacks had led some of the world’s largest shipping companies to have their ships avoid the waterway and take the longer routes around Africa.

The Iranian-backed Houthis have controlled parts of Yemen since 2014 and have demonstrated a willingness to expand their internal conflict beyond Yemen’s borders.

In October 2016, Houthi radar sites were targeted by U.S. airstrikes following anti-ship missile attacks targeting a U.S. Navy destroyer sailing in international waters.

This is a developing story. Please check back for updates.