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By Tomas Bravo

PIEDRAS NEGRAS, Mexico, April 25 (Reuters) - A tornado killed 10 people, injured at least 180 and left hundreds homeless when it struck along the U.S.-Mexico border and cut a four-mile (6.4-km) swath of damage.

Seven people were killed when the storm ripped through Eagle Pass, Texas, on Tuesday and at least three people died in Piedras Negras, Mexico, just across the Rio Grande, where schools, houses and churches were destroyed.

"Our hospital is just being overrun with people," Eagle Pass Mayor Chad Foster said on Wednesday after a night of frantic searches for the dead and injured.

Dogs, trees and the roofs of houses went flying through the air when the tornado's strong winds and driving rains pounded the twin border towns.

Mobile homes were knocked over in Eagle Pass, while an early warning system failed to go off in Piedras Negras, where 32 people died in floods in 2004, despite regular simulations, residents said.

"The tornado ripped off the roof and part of the wall of our house and trees fell down out front and out back. It was horrible," said housewife Juanita Rositas.

Five of the Eagle Pass dead were in a single residence, according to the mayor. A neighbor said they were members of a family whose mobile home was demolished.

All 1,500 residents of the Rosita Valley community south of Eagle Pass, where the tornado did the worst damage, were evacuated, said Maverick County Judge Jose Aranda.

PEOPLE IN SHELTERS

Between 250 and 300 people from the Texas community are living in shelters and there are "more coming all the time," he said.

In Piedras Negras, rescue officials said the army was searching for survivors and victims.

"Hundreds of houses are damaged, 700 or 800 people are in shelters and we've begun looking for people who may be trapped in the rubble," spokesman Fernando Horta of Coahuila state Civil Protection said.

Foster said the tornado obliterated the main Catholic church in Piedras Negras.

"There were two orphanages within proximity of that church. Our concern is for the young people who were in those orphanages," he said.

Aranda said wide sections of the stricken area had yet to be searched.

"We're doing a search and rescue operation with several police agencies, Border Patrol, National Guard, over 100 manpower," he said.

Many of the people in Rosita Valley have little beyond their homes and belongings and often lack insurance, Aranda said.

He said he planned to ask Texas Gov. Rick Perry, who was due to tour the area later on Wednesday, for state assistance to the victims.

(Additional reporting by Jim Forsyth in San Antonio and Gabriela Lopez in Monterrey, Mexico)

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