US forces have carried out a second phase of air strikes in response to the deaths of three American troops in Jordan last weekend, this time hitting targets in Yemen where Iranian-backed Houthi rebels have wreaked havoc on commercial shipping.

The US, joined by the UK, hit 36 Houthi targets in Yemen across 13 locations on Saturday, a day after it struck Islamic Revolutionary Guards Corps targets in Iraq and Syria, according to the Pentagon.

“Our aim remains to de-escalate tensions and restore stability in the Red Sea but let us reiterate our warning to Houthi leadership: we will not hesitate to continue to defend lives and the free flow of commerce in one of the world’s most critical waterways in the face of continued threats,” the US, UK and six other nations said in a joint statement.

Saturday’s strikes targeted Houthi weapons storage facilities, missile systems and launchers, air defences and radars, according to the statement. The US, sometimes acting with the UK, began striking Houthi targets on January 11 but the rebel group has continued its attacks on international vessels.

“This collective action sends a clear message to the Houthis that they will continue to bear further consequences if they do not end their illegal attacks on international shipping and naval vessels,” US Defence Secretary Lloyd Austin said in a statement.

Iran and Iraq warned earlier on Saturday that the US strikes could trigger greater instability across the region.

US President Joe Biden is trying to deter the continued targeting of American service members while avoiding being pulled into a full-on regional war. Since mid-October, Iranian-backed groups have launched more than 160 attacks on American troops in Iraq, Syria and Jordan.

Last weekend’s drone attack on a US military base in Jordan was the first to kill American forces since the war between Israel and Hamas began, raising the risk of escalation.

The US said it struck 85 targets at 7 sites on Friday. The Iraqi government said on Saturday that 16 people, including civilians, were killed in the attacks. The Syrian military said “many civilian and military martyrs” were killed, but did not provide any other details.

The US has also become more involved in the Red Sea, where Iranian-backed Houthis have launched more than 39 attacks on commercial and military vessels transiting the crucial waterway.

The US has already conducted more than 12 strikes in Yemen to deter the Houthis from launching more attacks, which the rebel group says will continue so long as Israel is attacking Gaza.

The most recent strike against Houthi targets, which officials described as opportunistic, took place earlier on Saturday when the US military hit six anti-ship cruise missiles it said were preparing to launch against ships in the Red Sea.

Washington blamed last weekend’s drone attack on the Islamic Resistance in Iraq, a shadowy umbrella group believed to include Kataib Hizbollah, an Iraqi Shia militia, as well as other militants that have claimed responsibility for the attacks against US troops in Iraq and Syria.

The IRI is part of Iran’s so-called Axis of Resistance, a network of militant groups which includes the Houthis in northern Yemen and Hizbollah in Lebanon.