The dead were lined up in formation when the missiles came. Three ballistic missiles fired at the al-Alam camp in Shabwa province - a police and army training facility in the Jardan district - killed at least 53 people and injured more than 200, most of them young recruits, in the deadliest houthi attack yemen has seen since the civil war reignited in September.

Three missiles on recruits in formation

The strike hit while recruits stood in formation, leaving the wounded to be driven from the camp to a hospital in Ataq, the provincial capital.

• •Three ballistic missiles struck the camp during the formation
• •The al-Alam camp trains both police and army personnel in the Jardan district
• •The injured were taken to a hospital in Ataq, where blood donations were requested

Al-Alimi vows response and UN action

Rashad al-Alimi, president of the UN-recognised government, said those killed were new recruits and vowed an immediate hard response. The killings, he said, "affirmed that targeting the recruits reflects the Houthi militias' approach of resorting to cowardly terrorist attacks whenever they face pressures or their Iranian-backed sabotage plots falter".

Al-Alimi had already phoned UN secretary general António Guterres on Monday to ask the UN to classify the Houthis as a foreign terrorist organisation.

Houthis say camp was run by Saudi officers

The Houthis described a different target. Their military spokesperson said its forces had struck "the Saudi enemy's mobilisations in the Al-Alam camp, which is run by Saudi officers in the eastern regions".

The Shabwa provincial authority said: "Targeting recruits constitutes a military escalation and a blatant assault on the province's security." Governor Awad bin Al-Wazir visited the wounded in hospitals, and an inquiry is to examine how so many troops were left so exposed to Houthi attacks.

A widening war

The camp strike is one front in a fast-moving escalation. In mid-September the Houthis began a fresh push south along the Red Sea coast, aiming to take control of the Bab al-Mandab waterway, and their forces have cut the road between government-run Shabwa and the Houthi-run Bayda governorate. They are also trying to mount a siege of Taiz, where citizens have called for greater air protection.

Reliable assessments of the death toll over the past month are hard to gather, but it is running into the many thousands. A Saudi airstrike on Sunday killed nine people, including five children and two women. Southern forces, meanwhile, were said to be making progress in edging Houthi forces out of the area around Bab al-Mandab, the strait between the Red Sea and the Indian Ocean.

The next diplomatic step falls to Hans Grundberg, the UN envoy for Yemen, who is due in London on Tuesday to meet ministers, while the inquiry into the troops' exposure is set to proceed.