Officials in Sierra Leone say at least three people were killed when a U.N. helicopter crashed into the sea late Wednesday near Freetown, the capital.

U.N. officials in Freetown say they have begun an investigation into the causes of Wednesday's tragedy. The world body's spokeswoman in Sierra Leone, Margaret Novicki, says there is no explanation yet for the crash of the Ukrainian-built MI8.

Ms. Novicki said the helicopter went down Wednesday while headed to the town of Lungi to pick up the head of Zambia's armed forces, Lieutenant General Geojago Musengule.

There were seven people aboard the aircraft: A four-member Ukrainian crew, two Zambian army officers and a U.N. volunteer from Bulgaria. The U.N. mission in Freetown says medical teams are trying to identify the three bodies that have been recovered.

The remaining four people are missing and presumed dead.

Sierra Leone is the site of one of the United Nation's largest missions. The world body has dispatched about 16,000 soldiers there to help end a brutal civil war between pro-government militias and rebels from the Revolutionary United Front.