BUENOS AIRES (Reuters) - Pope Francis was deeply grieved, the Vatican's official broadcasting service said on Tuesday, after the deaths of three relatives killed when their car slammed into the back of a truck on a highway in central Argentina.

A nephew of the Pope, Mr Emanuel Bergoglio, who was driving the car, was in serious condition following the accident just after midnight on Monday. His wife, 35, and two children aged two years and eight months died.

"The Pope has been informed and is deeply grieved by the tragic news. He asks all those who share his pain to join with him in prayer," said a statement on the website of Radio Vaticana.

The driver of the grains truck, Mr Raul Pombo, told local television stations he felt the impact and found Mr Bergoglio's vehicle wedged under the rear of his vehicle. "I began stopping passing cars to ask for their fire extinguishers because the car was on fire," Mr Pombo said.

The region's highway patrol chief, Mr Jorge Rainieri, called the impact "powerful" and television pictures showed the mangled wreckage of the compact car. The accident occurred in Argentina's central Cordoba province.