A cargo plane crashed into an empty area near the airport in Sudan's capital Khartoum on Monday, exploding into a fireball, witnesses said.

It was not immediately known how many people were on board the aircraft and whether they had survived or been killed.

"The plane was taking off from the airport ... It blew up into a fireball," said a U.N. official, declining to be named. He identified the aircraft as a Russian-built Ilyushin Il-76.

Police and emergency workers cordoned off the area and a Reuters cameraman said he had been briefly detained.

Witnesses at the crash scene said the plane appeared to have careered across a major road before exploding in an open space near an office building.

Wreckage was strewn across a wide area and firefighters were putting out the blaze, they said.

"There are huge patches of dark engine oil," said one witness.

Aviation officials could not be reached immediately for comment.