CAIRO -- Children accounted for a quarter of civilian casualties over the last three years in Yemen's grinding war between Iran-backed Houthi rebels and the internationally recognized government, a child relief agency said Tuesday.

More than 2,300 children were killed between 2018 and 2020, . However, the group said the actual toll is likely to be much higher.

"Yemeni children have been living through a horrific and endless nightmare for six years now. Children continue to be killed and injured on a near-daily basis," said the organization's country director, Xavier Joubert.

Save the Children's report comes on the heels of a UNICEF statement issued Saturday, saying that eight children were killed and 33 wounded this month alone.

Philippe Duamelle, UNICEF's representative to Yemen, said the casualties happened in several areas including the provinces of Taiz and Hodeida, where fighting between forces of the internationally recognized government and Houthi rebels has recently intensified.

On Monday, Saudi Arabia offered their Houthi rivals a cease-fire plan that includes the reopening of the international airport in the Yemeni capital of Sanaa, which has been under siege for almost six years. The Houthis downplayed the offer, claiming it did not offer anything new.

"All parties to the conflict must fully implement a cease-fire as soon as possible," said Joubert. "The cease-fire should be used to work towards a sustainable peace and a political solution to this war -- it's the only way to truly end this humanitarian catastrophe."

Like other relief agencies, the London-based Save the Children deplored plummeting funding levels for relief efforts in the war-stricken country. The group said its funds for aid to children in Yemen has dropped by more than 40% compared to last year.

"If the U.N.'s predictions are correct, the worst famine in decades could kill hundreds of thousands of children. We must do everything we can to prevent this from happening," said President and CEO Janti Soeripto.

Yemen has been convulsed by civil war since 2014 when the Houthis took control of the capital Sanaa and much of country's north, forcing the government of President Abed Rabbo Mansour Hadi to flee to the south, then to Saudi Arabia. A Saudi-led coalition, backed at the time by the U.S., entered the war months later to try to restore Hadi to power.

Now mired in stalemate, the war has killed about 130,000 people -- including more than 12,000 civilians -- and spawned the world's worst humanitarian crisis in a country that was already the Arab world's most impoverished nation.

The Biden administration has been keen to stop the war in Yemen. Earlier this month, the U.S. envoy for Yemen, Tim Lenderking, urged the Houthis to accept his cease-fire proposal. However, the rebels are still pushing aggressively to take the government stronghold of Marib, in central Yemen.

Also Tuesday, Human Rights Watch accused the Houthis of firing indiscriminate artillery and missiles into heavily populated areas as they push to take Marib. The New York-based group called on the rebels to halt their attacks and allow unimpeded humanitarian access to civilians trapped by the fighting.

Afrah Nasser, the group's Yemen researcher, said the Houthis have "shown a shocking disregard for the well-being and safety of civilians." A spokesman for the Houthis was not immediately available for comment.

Last week, a Houthi missile reportedly hit a fuel station at a Marib market, killing at least two civilians and wounding seven.

"It was tragic seeing blood and flesh of bodies spread on the ground and the damaged cars ... The explosion was huge, and we felt it across all Marib city," HRW quoted an unnamed witness as saying.

The group said Houthi artillery and heavy weapons have hit several camps for displaced people last month in the city. Already, four displacement camps have been abandoned since the start of the offensive, the U.N. migration agency said.

Houthi military spokesman Brig. Gen. Yehia Sarie, meanwhile, said they targeted Saudi Arabia's southwestern Abha airport with a bomb-laden drone.

There was no immediate comment from the Saudi-led coalition on the attack, which came a day after Saudi Arabia announced a cease-fire initiative in Yemen.

Sarie also claimed they shot down a U.S. MQ-9 drone over Marib. He did not say which country operated the drone.

Neither claim could be immediately confirmed.

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Associated Press writer Samy Magdy in Cairo contributed to this report