DENVER (AP) — Tension over the drought-stressed Colorado River escalated into a public feud when four U.S. states accused an Arizona utility of manipulating supply and demand, potentially threatening the river's future.

The four states said the Central Arizona Project was trying to avoid a reduction in its share of the river while others are voluntarily conserving to avoid a crisis amid a prolonged drought.

The accusation was made by Colorado, New Mexico, Utah and Wyoming, along with Denver's water utility.

The Central Arizona Project denied the allegations. The project serves Phoenix, Tucson and other cities.

The Colorado River supplies water to nearly 40 million people in the U.S. and Mexico. It's under increasing strain because demand is rising while the river is shrinking.

Researchers blame an 18-year drought and climate change for the decline.