"I would like to express sincere condolences to all the families who have lost loved ones, in particular their daughters who were in a summer camp in the disaster caused by flooding of the Guadalupe River in Texas. We pray for them," said the US-born pontiff following Angelus prayers.

Rescuers searched through the night to try to locate 27 girls and teenagers missing from a riverside summer camp after flash floods caused by torrential rains on Friday, when the Guadalupe River rose eight metres (26 feet) in just 45 minutes.

Nearly 300 millimetres of rain per hour suddenly fell, a third of the average annual rainfall.

Later Sunday, the Pope travelled to Castel Gandolfo, the summer papal residence some 20 kilometres (12 miles) south-east of Rome where he will take a two-week break.