Rebels controlling Yemen's capital called on the United Nations on Friday to take action to end violence that has destroyed large parts of the country and to investigate bombings carried out by a Saudi-led coalition.

In a letter to U.N. Secretary-General Antonio Guterres, the Houthi movement urged the U.N. to investigate a Saudi-led airstrike on a funeral in Sanaa last year and not renew the term of U.N. peace envoy Ismail Ould Cheikh Ahmed.

"The United Nations should take a serious stance against the foreign aggression in Yemen and end the air and naval blockade imposed by Saudi Arabia," the letter quoted Saleh al-Sammad, a Houthi official, as saying.

The statement is a blow to the U.N., which has sought since 2015 to end fighting between the Iran-aligned Houthis and a Saudi-led alliance of mainly Gulf states in a conflict that has unleashed mass hunger and disease and killed over 10,000 people.

Ould Cheikh Ahmed, who has served as United Nations Special Envoy for Yemen since April 2015, has brokered several ceasefires, which have however foundered within days.

Earlier this week, Yemeni government forces backed by Gulf Arab troops recaptured control of the Red Sea city of al-Mokha in a push that paved the way for an advance on Hodeidah, the country's main port city.

The U.N. said on Friday that the Saudi-led coalition had intensified air strikes on Hodeidah, possibly trapping civilians and hampering a humanitarian operation to import vital food and fuel supplies.

The statement also expressed concern that civilians in al-Mokha had been deliberately targeted by Houthi-linked gunmen during the battle for control of the port.

Across Yemen, some 12 million people, roughly half the population, face the threat of famine and conditions are worsening, the United Nations warned on Wednesday as it appealed for $2.1 billion to fund food and other life-saving aid.