SANAA, Yemen -- Yemen's prime minister on Tuesday called on the Saudi-led coalition allied with his government to deposit $1 billion in the central bank to save the local currency from "complete collapse."

In an emotional letter, Ahmed Obeid bin Daghir called on the coalition to act "now, not tomorrow," and said saving the rial means "saving Yemenis from inevitable famine."

The rial, now trading at 500 to the dollar, has lost half its value since the coalition went to war against Yemen's Houthi rebels in 2015. Food and fuel prices have soared, pushing many to the brink of famine.

The UN has called Yemen the world's worst humanitarian crisis. The war pits the coalition and an internationally recognized government against the Houthis, rebels allied with Iran who control much of northern Yemen, including the capital, Sanaa. The war, which has been locked in a bloody stalemate for most of the last three years, has killed an estimated 10,000 people and displaced 2 million.

On Tuesday, protests broke out in the city of Taiz, which has been split between the Houthis and pro-government forces for the past three years. Hundreds of protesters on the government-held side set fire to pictures of Daghir and the governor of the central bank, and chanted against the coalition.

A senior Yemeni official told The Associated Press that the coalition hasn't moved to shore up the central bank because of mistrust between the United Arab Emirates, a key member of the alliance, and Yemen's President Abed Rabbo Mansour Hadi. The official spoke on condition of anonymity because he wasn't authorized to speak to the press.

The UAE is believed to be at odds with Hadi over his embrace of a local affiliate of the Muslim Brotherhood group, and fighters loyal to the two sides have clashed in recent months. Hadi has been prevented from returning to Yemen by the Saudis following his fallout with the UAE.

Last August, the governor of the central bank, Monasser al-Quaiti, said the coalition had blocked 13 flights bringing cash into the country and was "strangling" its economy.

The government moved the central bank from Sanaa to the southern port city of Aden last year, saying the Houthis were using it to finance the war. In Houthi-run areas, hundreds of thousands of civil servants have not been paid in more than a year.

The economic collapse has contributed to the breakdown of basic services, fueling a cholera outbreak that has killed some 2,000 people and infected a million.

The UN children's agency said Tuesday that more than 5,000 children have been killed or wounded in Yemen's war, an average of five per day since March 2015. It said nearly every child in Yemen requires humanitarian assistance, and that 1.8 million are acutely malnourished.