Yemen's prime minister has 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 released Tuesday, Ahmed Abdu 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 U.N. has called Yemen the world's worst humanitarian crisis.