(ADAMUZ, Spain) -- At least 40 people have been killed and more than 100 others were injured after two high-speed trains collided in southern Spain, according to emergency officials.
A train traveling from Málaga to Madrid on Sunday derailed near Adamuz, crossing over to the adjacent track where it hit another train coming from Madrid to Huelva, according to the Spanish Interior Minister.
Eighty-one of the injured have been discharged and 41 remain hospitalized, emergency officials said on Monday. Twelve of the hospitalized victims are in the intensive care unit, officials said.
An unknown number of people remained unaccounted for on Monday as rescue crews continued to work at the scene, according to a Spanish official.
Regional President of Andalusia Juan Manuel Moreno said rescue crews are working through difficult conditions to try to reach the train carriages, where more victims could be inside.
About 400 people were on board both trains, officials said.
Oscar Puente, the Spanish transport minister, said in a statement early on Monday that the death toll was "not final."
"I want to express all my gratitude for the huge effort of the rescue teams during the night, under very difficult circumstances, and my condolences to the victims and their families in these terribly painful moments," he said in Spanish on social media.
The cause of the train derailment has not been released.
Iryo, the company operating the train that initially derailed, released a statement, saying the company "deeply regrets what has happened and has activated all emergency protocols, working closely with the competent authorities to manage the situation."
Puente, the transport minister, said the high-speed Iryo train was "relatively new."
Puente said the derailment of the Iryo train bound for Madrid and its subsequent collision with the second train happened on a straight stretch of track, which had undergone extensive renovation work that was only finished in May.
The Spanish minister called the accident "extremely strange."
"It’s very difficult at this moment to explain," Puente added, and said he hoped the investigation would help clear up what has happened.
ABC News' Joe Simonetti contributed to this report.