KAMPALA, Uganda — At least six people were killed and dozens more injured when a four-story building under construction collapsed in eastern Rwanda, police said Wednesday, as the country's military joined the police to search for survivors under the rubble.

In a statement police described the Tuesday afternoon incident in the eastern Rwanda district of Nyagatare as "an unfolding tragedy," but it was not clear what caused the collapse or even how many people are feared buried under the rubble.

Christophe Semuhungu, the police spokesman for Rwanda's Eastern province, said Wednesday that at least 30 workers were injured in the collapse. Most were being treated for minor injuries at a nearby hospital, he said, but some were in critical condition. He said the construction site employed about 45 workers although it was unlikely all of them were on duty when the building fell.

"The situation is now contained," he said. "The action of searching has taken off."

In recent years building collapses have become frequent in East and Central African countries as some property developers bypass regulations to cut costs. In March, a building collapsed in the Tanzanian city of Dar es Salaam, killing at least 30 people.