A fireman holds a child who was rescued from the rubble of a school collapse in Haiti on Friday.
Clarens Renois, a journalist with the Haiti Press Network, said nearly 200 children remained trapped under rubble as of midnight Friday.
As many as 700 children were inside when the building collapsed around 10 a.m. ET Friday, officials said. Some were in class and others were in a playground, Haitian media reported.
"We are looking at major casualties here," said Alex Claudon, a Red Cross official on the scene.
President Rene Preval and Prime Minister Michele Pierre-Louis toured the disaster area. The Haiti Press Network quoted Preval as saying that he "heard and saw with my own eyes children appealing for help."
At least one member of the Haitian Parliament has raised questions about whether the school was built for the number of students and teachers who were inside when the College La Promesse Evangelique collapsed, Renois said. The official described the building as "not quite solid" with "weak construction." Watch rescuers pull students from the rubble »
Preval has since called for a review of building-construction guidelines.
The Haitian Civil Protection Bureau said at least 100 people have been injured, but the death toll is expected to rise.
Most of the students at the school ranged in age from 10 to 20, officials said, but some are younger. Haitian press reports said kindergarten, primary and secondary students attended the school.
"We are taking all necessary steps. The government has mobilized to save those who can be saved," Pierre-Louis said.
Preval asked residents to stay away from the area to allow police and rescue officials to do their work unimpeded.
Michaele Gedeon, president of Haiti's Red Cross, said that, while she was on the phone with rescuers trying to coordinate their efforts, she could hear the voices of distraught children.
"On the phone you can hear so many children, you know, crying, crying, and saying, "This one is dead. that one is dead,'" she said.
In a later interview, Claudon said, "Local authorities are doing their best."
About 50 to 60 patients, 30 of them severely injured, were taken to Trinite Hospital in Port-au-Prince, said Isabelle Mouniaman Nara, the head of mission in the capital for Doctors Without Borders.
Another 150 patients were treated elsewhere, Nara said Friday night.
The situation at Trinite "is under control right now," she said.
Trinite is the only hospital that is open in Port-au-Prince, Doctors Without Borders said. The other two -- General Hospital and Hospital de la Paix -- have been shut down due to worker strikes.
The school is in an extremely poor part of town and the roads are nearly impassable, Renois said. He also said an United Nations helicopter was unable to land.
"The school is poorly built," said Amelia Shaw, a journalist with United Nations TV who visited the scene.
The two-story school had an addition built in the rear over a 200-foot ravine, Shaw told CNN by telephone. The steep hillside, she said, is covered with run-down houses and shacks on both sides.
The disaster occurred when the second floor crumbled onto the first, Shaw said.
The U.S. Agency for International Development sent a Disaster Assistance Response Team, which arrived on the scene within hours of the collapse, the agency said in a news release.
After assessing the situation, USAID activated its partner, the Fairfax County Urban Search and Rescue Team. That team will be composed of 38 personnel, four search-and-rescue dogs and 31,000 pounds of rescue equipment and is expected to arrive Saturday.
The Urban Search and Rescue Team will be accompanied by four additional USAID disaster experts.