Eighteen people gathered in a mountain hostel in eastern Spain for a birthday party died in their sleep from an apparent gas leak, reports said Sunday.

The victims — ages 20-40 — were part of a group of 50 people from several villages who converged on a hostel in the hamlet of Todolella for Saturday night's party.

Most went home afterward but 18 spent the night in the hostel and lit at least one gas heater in the room where they were to sleep, reports said.

The heaters either leaked butane gas or did not burn properly and instead released deadly carbon monoxide, Efe and Europa Press said, quoting police.

Police in Castellon confirmed the deaths but said they had no further details.

The newspaper El Mundo's Web site quoted town hall officials and the Civil Guard as saying the bodies were found late Sunday afternoon.

The hostel is set in the mountains and is frequented by bikers, hikers and horseback riders, the Web site said. Castellon is part of the Valencia region.

Valencia's regional president, Francisco Camps, and senior ministers were rushing to the scene of the accident, national radio said.