The president went on to offer "heartfelt gratitude" to Spanish officials and rescue personnel who are working to locate the missing and treat the injured – including at least six Americans."

"Today the American people grieve with our Spanish friends, who are in our thoughts and prayers," Obama said. "We stand ready to provide any assistance we can in the difficult days ahead."

The train crash was Europe's worst mainline rail accident in more than 25 years. The driver has been put under formal investigation.