Saudi-led air strikes on Yemen killed at least 31 civilians on Saturday, the United Nations said, an apparent retaliation after a Saudi fighter jet crashed with Iran-backed Huthi rebels claiming to have shot it down.The Tornado aircraft came down Friday in northern Al-Jawf province during an operation to support government forces, a rare crash that prompted air strikes in the area by a Saudi-led military coalition fighting the rebels.The deadly violence follows an upsurge in fighting in northern Yemen between the warring parties that threatens to worsen the war-battered country's humanitarian crisis."Preliminary field reports indicate that on 15 February as many as 31 civilians were killed and 12 others injured in strikes that hit Al-Hayjah area... in Al-Jawf governorate," said a statement from the office of the UN humanitarian coordinator for Yemen.The UN humanitarian coordinator for Yemen Lise Grande denounced the "terrible strikes"."Under international humanitarian law parties which resort to force are obligated to protect civilians. Five years into this conflict and belligerents are still failing to uphold this responsibility. It's shocking," she said.The rebels said women and children were among the dead and wounded in retaliatory coalition air strikes.The coalition conceded the "possibility of collateral damage" during a "search and rescue operation" at the crash site.A coalition statement released by the official Saudi Press Agency did not specify the fate of the crew or the cause of the crash.'A major blow'But the Huthi rebels released footage of what they called the launch of their "advanced surface-to-air missile" and the moment it struck the jet in the night sky, sending it crashing down in a ball of flames."The downing of a Tornado in the sky above Al-Jawf is a major blow to the enemy and an indication of remarkable growth in Yemeni (rebel) air defence capabilities," Huthi spokesman Mohammed Abdelsalam tweeted.The insurgents reported multiple coalition air strikes on Saturday in the Huthi-controlled area where the plane went down as local residents gathered near the smouldering wreckage, according to the rebels' Al-Masirah television.The escalation follows fierce fighting around the Huthi-held capital Sanaa, with the rebels seen to be advancing on several fronts towards Al-Hazm, the regional capital of Al-Jawf.The province of Al-Jawf has been mostly controlled by the Huthis, but its capital remains in the hands of the Saudi-backed government.'Massively expanded arsenal'The downing of a coalition warplane marks a setback for a military alliance known for its air supremacy and signals the rebels' increasingly potent military arsenal."At the start of the conflict the Huthis were a ragtag militia," Fatima Abo Alasrar, a scholar at the Middle East Institute, told AFP. "Today they have massively expanded their arsenal with the help of Iran and its proxy Hezbollah," Lebanon's powerful Shiite movement.Huthi rebels now possess weapons bearing signs of Iranian origin, according to a UN report obtained by AFP earlier this month, in potential violation of a UN arms embargo.Some of the new weapons, which the rebels have possessed since 2019, "have technical characteristics similar to arms manufactured in the Islamic Republic of Iran," said the report, compiled by a panel of UN experts tasked with monitoring the embargo.The panel did not say whether the weapons were delivered to the Huthis directly by the Iranian government, which has repeatedly denied sending them arms.The coalition intervened against the Huthis in 2015, in a conflict that has killed tens of thousands of people, most of them civilians, and sparked what the United Nations calls the world's worst humanitarian crisis.The coalition has been widely criticised for the high civilian death toll from its bombing campaign, which has prompted some Western governments to cut arms deliveries to the countries taking part.Saudi Arabia and the UAE have purchased billions of dollars' worth of weapons from the United States, France and Britain.On Wednesday, the coalition said it would put on trial military personnel suspected of being behind deadly air strikes on Yemeni civilians.