GHOTKI, Pakistan (AP) An express train barreled into another that had derailed in Pakistan before dawn Monday, killing at least 51 people and setting off a desperate effort to search the crumpled cars for survivors and the dead, authorities said.

More than 100 other people were injured. Cries for help pierced the night as passengers climbed out of overturned or crushed rail cars. The pleas continued to echo throughout the day at the scene in the district of Ghotki, in the southern province of Sindh.

Heavy machinery arrived to cut open some cars, and more than 15 hours after the crash, rescuers carefully removed wreckage as they looked for anyone who might remain trapped though hopes were fading for survivors. The military deployed troops, engineers and helicopters to assist.

The Millat Express train derailed around 3:30 a.m., and the Sir Syed Express train hit it minutes later, said Usman Abdullah, a deputy commissioner of Ghotki. It was not immediately clear what caused the derailment, and the driver of the second train said he braked when he saw the disabled train but did not have time to avoid the collision.

About 1,100 passengers were aboard the two trains, rail officials said.

The challenge for us is to quickly rescue those passengers who are still trapped in the wreckage, said Umar Tufail, a police chief in the district.

The death toll steadily rose through the day, and the chances of finding survivors were diminishing, said Rizwan Nazir, a district administration official.