PETION-VILLE, Haiti - About 50 schoolchildren and teachers were killed when a shantytown school packed with hundreds of students collapsed during classes Friday, a government official said.

The three-story La Promesse (The Promise) school in Petion-ville, on the outskirts of Port-au-Prince, caved in in a heap of cement slabs and twisted steel rods at about 10:00 am (1500 GMT) Friday, trapping scores inside.

A new story had been under construction atop the school when it fell in, also destroying or damaging five homes next to it.

"We have counted about 50 dead for the moment, and around 85 injured," said Nadia Lochard of the civil protection bureau.

"But there are still numerous children stuck in the rubble. We have signs that they are still alive and we are organizing help to try to save them," she said.

Lochard said that French fire crews from the island of Guadeloupe were to arrive overnight Friday to help in the rescue.

In Washington, the US Agency for International Development (USAID) said search and rescue teams were en route to the scene, due to arrive Saturday morning.

"This is a tragic situation, especially since children are involved. We are working alongside the Haitian government to provide immediate assistance in the rescue efforts," said USAID Administrator Henrietta Fore.

In Paris, French Foreign Minister Bernard Kouchner expressed his "deep emotion" over the tragedy and promised to send "as soon as possible a civil security team to help the Haitian authorities in rescuing the victims who are still buried."

As many as 700 students aged from three to 20 attend the church-run school in a suburb of the capital, but an accurate count of how many had been inside when it crumbled was not available.

Frantic parents rushed to the site in search of their children after the building caved in, and scores of people climbed over the pile of crumbled concrete-and-steel bar to rescue those pinned underneath, their faces covered in the grey dust of the cement.

UN engineers and soldiers from the UN mission in Haiti (MINUSTAH) worked to remove heavy pieces of concrete and to contain the large crowds of people gathering at the school.

Haitians carry a victim of the College Promesse Evangelique school collapse in Petion-ville, Haiti
?AFP - Thony Belizaire

Cries of distress could be heard around the site, from still-alive students and teachers beneath the rubble and from parents desperately searching for their children.

The bodies of the first victims, some with their limbs crushed, were laid inside a building next to the school and covered with sheets.

A deeply distressed Marie Flore said she had no news of any of her three children.

"It brought down the rest of the building while the students were in class," said another panic-stricken woman who had hurried to the scene to look for her child.

The International Red Cross, the Haitian Red Cross, members of the UN Haiti peacekeeping force and other groups joined in the rescue.

Haitian President Rene Preval and Prime Minister Michele Pierre-Louis hurried to the scene to oversee rescue efforts, as other officials pledged an investigation into the construction and a survey of other schools possibly at risk.

"This construction did not meet normal standards. We are going to ask the minister of education to make an inspection of all the schools built in the same way," Senator Yvon Bissereth told AFP.

"What we need right now is heavy search and rescue equipment to come here."

The most seriously injured people were being ferried to the closest hospitals in ambulances provided by the Red Cross, Doctors Without Borders and the UN mission in Haiti.