The tank exploded at about 8:30 a.m. at the Red Snapper restaurant, located about two blocks from a bridge over the Rio Bravo connecting Mexico with the United States.
The three-story cement building and a neighboring pharmacy and souvenir shop were demolished in the blast, which shattered windows up and down the street. Investigators said the explosion appeared to be accidental.
``It's not clear exactly what caused the explosion, but it's known it was a gas leak,'' said Jose Cerecedo, director of the public security for Nuevo Progreso, a town of 15,000 on the U.S. border at Texas, and parent city Rio Bravo.
At least six people were killed, Mexican police officer Raul Herrera Soto said. Four injured people were hospitalized; their conditions were not immediately available.
The restaurant was not open when the tank exploded on the second floor, but employees were in the building, said George Garrett of the Weslaco, Texas, emergency management team.
Officials called a news conference in the evening to clear up rumors about terrorism.
``It's not clear what it was that happened _ but it was not a bomb,'' said Rio Bravo City Council President Alvaro Cervantes.
It was unclear how many people were still trapped in the rubble.
Francisco Martinez said the blast killed his sister-in-law, a U.S. citizen who lived and worked in Nuevo Progreso.
``It looked like a war zone, it was awful,'' said Martinez, who found the body of Natividad Avalos, 25, a secretary at the dentist office. ``We were able to get my sister-in-law's body because she was just blown out of the building.''
A four block area was cordoned off by authorities, and the McAllen, Texas, Fire Department used rescue dogs and thermal imaging equipment to search for survivors.
``It's kind of like a puzzle,'' said Ramon Garcia, a judge in neighboring Hidalgo County, Texas, who was helping coordinate U.S. emergency crews. ``We need to be careful, otherwise we're going to be hurting people who are trapped inside.''
The bridge from Mexico to Progreso, Texas, was closed because of the explosion, though rescue workers from the United States were allowed to enter Mexico.
The city along the Rio Bravo river _ called the Rio Grande in the United States _ is a popular destination for tourists from South Texas who flock to the area's restaurants, bars and shops.
Witnesses said the ground shook and a thick cloud of dust filled the air.
``It was a loud boom and we heard the echoes. It kept echoing,'' said Jose Ramirez, who owns a fabric shop across the street and heard the explosion from his home four blocks away.
Crowds still were sparse at the time because of the early hour, local police spokesman Enrique Hernandez said.
``Normally, there's a big flow of people,'' Hernandez said. ``Maybe that's why more people were not injured. We hope we don't locate any more bodies.''
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