At least 31 people have been killed and 101 injured after the United States launched a series of airstrikes on Yemen on Saturday, according to the Houthi health ministry, as a rebel spokesperson told NBC News that they group would respond with “more escalation.”

President Donald Trump said he ordered the airstrikes on Houthi-held areas of Yemen on Saturday and promised to use “overwhelming lethal force” until the rebel group ceased attacks on shipping along a vital maritime corridor.

The Houthis, an Iran-backed militia that has taken over part of Yemen, including its capital, Sanaa, began launching missiles and drones and pursuing other attacks against shipping vessels in response to the war in Gaza.

The group reported explosions Saturday evening in Sanaa, and Saada, the Houthi stronghold near the Saudi border, with more airstrikes early Sunday. Images online showed black smoke over Sanaa’s airport complex, which includes a military facility. Strikes were also reported in Hodeida, Bayda, and Marib.

The attack came a few days after the Houthis said they would resume attacks on Israeli vessels sailing off Yemen in response to Israel’s latest blockade on Gaza, which has halted the flow of aid into the enclave. There have been no reported attacks since this threat was made.

Nasr el-Din Amer, a spokesperson for the Houthis, told NBC News on Sunday that most of the casualties from the airstrikes were women and children, which NBC News has not independently verified.

Amer said the Houthis would continue their operations “until the blockade on Gaza is lifted.”

“We will respond to the recent escalation with further escalation,” he said. “Our response will not be delayed.”

Trump also warned Iran to stop supporting the rebel group, and promised hold the country “fully accountable” for the actions of its proxy Houthi group. Tehran supports the Houthis with money, training and equipment.

In a statement to Iranian state media, Hossein Salami, the commander of Iran’s Revolutionary Guard Corps, said Iran would “respond decisively and destructively” to any enemy taking threats into action.

But Salami also distanced Iran from the Houthis emphasizing that the group made their strategic decisions independently.

“The Islamic Republic of Iran plays no role in shaping the national or operational policies of any resistance front group,” he said, in remarks that could be interpreted as a signal that the Houthis are on their own.

An annual $1 trillion flow of goods passes through the Red Sea.

The Houthis have targeted over 100 merchant vessels with missiles and drones, sinking two vessels and killing four sailors during their campaign targeting military and civilian ships between the start of the war between Israel and Hamas in late 2023 and January of this year, when the current ceasefire in Gaza took effect.

Some shippers responded to the attacks late last year by suspending service in the region.

Earlier this month, Secretary of State Marco Rubio redesignated the Houthis as a foreign terrorist organization. The designation had been revoked by Joe Biden’s Democratic administration over concerns it would badly affect the delivery of aid to Yemen, which was considered to be facing one of the world’s worst humanitarian crises.