At least 53 Yemeni soldiers were killed and 200 wounded when Houthi rebels hit a military training camp with missiles in the southern province of Shabwa on Monday, Yemeni military officials said. They said it was one of the deadliest strikes since fighting intensified this year.

The Iran-backed rebels fired ballistic missiles at a camp run by forces of the internationally recognised government, which Riyadh backs. A Yemeni officer said the missiles landed as new recruits were lining up for training. The Houthis said they carried out the attack. They claimed Saudi officers ran the camp and that some of them were killed, but gave no proof.

Local authorities in Shabwa condemned the strike as a serious escalation against recruits who are still in training. They warned that the death toll could rise because the count is still going on. Provincial officials urged the presidential leadership and the Saudi-led coalition to deploy air defence and early warning systems there faster.

Rashad al-Alimi, head of Yemen's Presidential Leadership Council, promised a "firm response". He said the Houthis turn to terror attacks when they are under military pressure. Saudi officials did not comment immediately.

Saudi Arabia has called off a major international investment conference planned for next week because of repeated Houthi attacks. Senior figures from Wall Street and other international financial institutions were expected to attend.

On Monday the Houthis announced six more attacks on Saudi Arabia, targeting oil sites, military facilities and Riyadh's international airport. They did not say when the attacks took place. Saudi civil defence issued danger warnings. There were no immediate reports of deaths or damage from the latest attacks.

On Sunday the health ministry, which the Houthis control, blamed Saudi forces for an air strike in the northern province of Saada. At least 9 people died there, including 5 children and 2 women. Riyadh did not respond immediately. The Houthis say strikes over the weekend killed or wounded more than 30 people.

The Houthis hold the capital Sanaa and much of northern Yemen. Saudi Arabia joined the war in 2015 after the rebels seized Sanaa. A truce in 2022 largely stopped heavy fighting, but the civil war flared up again in February, when a wider conflict involving Iran began.

In recent weeks the Houthis have hit Saudi airports, oil facilities and military sites. They have also seized areas along the Red Sea coast near the Bab el-Mandeb Strait, the sea passage between the Red Sea and the Gulf of Aden. Earlier this month the coalition launched an offensive along the coast to push them out of the newly seized areas.

UN experts say Iran and allied groups in Lebanon and Iraq give the Houthis military expertise, training and advanced weapons. Tehran denies supplying them with arms. Riyadh has for years seen the Houthis' military strength and their ties to Iran as a threat to its security.

The US State Department on Monday again called for the immediate and unconditional release of more than 20 local staff of the US embassy in Sanaa, whom the Houthis have held for five years. The embassy stopped work in 2015, but the staff still provide security and maintenance.