A nurse checks a man who was injured by an airstrikes while attending a wedding in a village in northwestern Yemenon April 23. (Reuters)

An airstrike by the Saudi-led coalition fighting in Yemen’s civil war killed more than 20 people at a wedding party late Sunday night, Yemeni officials said.

Most of the victims of the attack in northwestern Yemen appeared to be children, according to officials and photos posted by victims and their families on social media sites. In televised footage, widely shared online, a crying child in a green shirt is seen clutching the body of his father, refusing to allow responders to take them to a hospital.

“I will not let go,” the boy screams repeatedly.

The attack comes after a top political leader for the Houthis, the rebel movement that controls much of northern Yemen, was reportedly killed in an airstrike last week. The rebels vowed Monday to avenge the death of Saleh Ali al-Sammad, chairman of their Supreme Political Council, who wielded great influence in the movement.

“The assassination of al-Sammad will not go unpaid,” said Mohammed Abdul-Salam, a Houthi spokesman, on the rebels’ television channel, al-Masirah.

More than 55 people were wounded in the attack in Alraqqah, a remote village in Hajjah province, health officials said. The death toll is expected to rise.

“There are many children in critical and severe conditions, and [they] have been put in intensive care and operation rooms,” said Mohammed al-Sowmali, general manager of al-Jamhouri hospital, where most of the victims were taken. “Four children have had at least one of their limbs amputated.”

It was the third deadly attack on civilians in recent days. On Sunday, an airstrike hit a house, killing a family of five, and on Saturday, at least 20 people died in an airstrike that hit a commuter bus near the southwestern city of Taiz.

The three-year-old war pits the Shiite Houthis against a regional coalition of Sunni powers led by Saudi Arabia. The coalition, backed by the United States and other Western powers, is seeking to restore the government of President Abed Rabbo Mansour Hadi, whom the Houthis drove from the capital, Sanaa, early in 2015.

The humanitarian crisis has only worsened in Yemen, the Middle East’s poorest country. More than 10,000 people have been killed, and countless more wounded, while an estimated 3 million people have been forced to flee their homes. A hunger crisis and epidemics of cholera and other diseases have gripped the country.

The conflict is widely considered a proxy war between the Iran-backed Houthis and Saudi Arabia, the Shiite theocracy’s main regional rival.

An official with the Saudi-led coalition was not reachable for comment, but in the past the coalition has said they do not target civilians. They have launched their own internal investigations into alleged civilian casualties. But the United Nations and Western relief agencies have found that of the thousands of civilians killed in the past three years, most died as a result of airstrikes by the coalition.

On Monday, U.N. Secretary General António Guterres condemned the attack on the wedding and urged all parties in Yemen’s war to observe international laws established to protect civilians in armed conflicts.

He called “ for a prompt, effective and transparent investigation,” said U.N. spokesman Stephane Dujarric in a statement.