RIYADH (Reuters) - A Saudi-led coalition air strike last week killed the top civilian leader in the armed Houthi movement in Yemen, the group reported on Monday, the most senior official to be killed by the Western-backed alliance in the three-year-old war.

FILE PHOTO: Saleh al-Sumad, the head of Yemen's Houthi movement's politburo, delivers a speech at a ceremony during which a political council to rule the country was announced in Sanaa, Yemen August 6, 2016. REUTERS/Khaled Abdullah/File Photo

Saleh al-Samad held the post of president in the Houthi-backed political body which runs most of northern Yemen.

Houthi leader Abdul Malik al-Houthi said in a televised statement that Samad was killed on Thursday in the port city of Hodeidah, on Yemen’s west coast, in several strikes which killed six others in his retinue.

“The forces of aggression, led by America and Saudi Arabia, bear the legal responsibility for this crime and all its consequences,” al-Houthi said.

There was no immediate coalition comment on the reports but Saudi broadcaster Al Arabiya TV said the alliance had killed Samad after a “precise monitoring” of his movements.

At the United Nations meanwhile, U.N. Secretary-General António Guterres strongly condemned air strikes on a wedding party in Yemen that killed at least 20 civilians on Sunday.

Saudi Arabia and its mostly Gulf Arab allies accuse the Houthis of being proxies of their arch-rival Iran -- charges the group and Tehran deny.

The Houthis control the capital Sanaa and are fighting forces loyal to the internationally-recognized government based in the southern port city of Aden. The government is backed by a mostly Gulf Arab alliance which in turn receives arms and other support from the United States and Britain.

Samad was second on the coalition’s most wanted list of Houthi leaders, after al-Houthi. It had offered a $20 million reward for any information that led to Samad’s capture.

The death of Samad, the group’s Ansarullah political council chief, dealt one of the biggest blows yet to the Houthi in a war that has killed more than 10,000 people and unleashed the world’s most urgent humanitarian crisis.

The United Nations has been advancing efforts to bring the warring sides closer together and achieve a peace deal to spare the impoverished country further bloodshed.

“This can potentially escalate the conflict, as it comes comes amidst tense political negotiations,” said Adam Baron of the European Council for Foreign Relations.

“The Houthis will feel the need to respond,” he added.

Al-Masirah reported that the Houthis had appointed Mahdi al-Mashat, once director of al-Houthi’s office, to replace Samad.

Residents and medical sources said coalition air strikes also killed at least 20 people attending a wedding in a village in northwestern Yemen on Sunday night, and about 30 people were wounded. The coalition said it would investigate the report.

The United Nations’ Guterres also called for a prompt and transparent investigation.

“The Secretary-General reminds all parties of their obligations under international humanitarian law concerning the

protection of civilians and civilian infrastructure during armed

conflicts,” a U.N. statement said.

The coalition has carried out thousands of air strikes in Yemen that have hit schools, markets and hospitals, killing hundreds of people, although it says it does not target civilians.