The three-storey building caved in on Friday morning during class, destroying neighboring homes and leaving scores of students and teachers trapped beneath huge slabs of cement and twisted steel rods. Some 107 were injured.

As many as 700 students aged between three and 20 attended the church-run school on the outskirts of the capital Port-au-Prince, though it was unclear how many were inside when the building came crashing down.

As the death toll continued to rise, Haitian President Rene Preval warned that there was no telling how many more bodies might be found.

'This morning we found a classroom with 21 inside - students and their professor - and they are all dead,' said Mr Preval, who along with Prime Minister Michele Pierre-Louis had rushed to the scene to oversee the rescue effort.

'Right now we cannot say how many bodies we will have because the work is not yet finished.' At the time of the collapse, builders had been adding a new floor atop the La Promesse school in Petion-ville, officials said.

Frantic parents crowded the site as scores of people, their faces covered in grey dust, climbed over the debris to try and rescue those pinned underneath.

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.

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.

A deeply distressed Mrs 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.

By late Friday some 50 bodies, most of them children, had been found. But the toll kept rising.

More children could still remained trapped beneath the rubble, Ms Nadia Lochard, a spokesman from the civil protection bureau told AFP early on Saturday.

At the hospital run by Doctors Without Borders, medics worked around the clock to save the injured. Eight patients died during the night, but there were success stories.

'Two seven-year-old children - a boy and a girl - have been saved,' Ms Lochard said. 'One of them underwent a successful operation,' she said, without elaborating.

In Washington the US Agency for International Development (USAID) said search and rescue teams were on their way to the scene.

'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,' USAID administrator Henrietta Fore said.

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

Other officials pledged an investigation into the construction of the school and a survey of other education establishments that might be at possible 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 said. -- AFP