A cargo plane crashed shortly after taking off from Khartoum airport on Monday, exploding into a fireball and killing all four crew, witnesses and officials said.

It was the fourth fatal air accident in Sudan in two months.

"The accident happened less than a kilometre (half a mile) from the airport, just seconds after it took off. Four of the crew, all of them Russians, were killed," said Abdel Hafiz Abdel Rahim, spokesman for Sudan's civil aviation authority.

He said the plane was carrying goods to the southern town of Juba for Ababeel Aviation, a private cargo company.

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

Witnesses at the crash scene said the plane appeared to have careered across a major road and exploded 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.

Police and emergency workers cordoned off the area and a Reuters cameraman said he had been briefly detained. Monday's crash was the fourth deadly aviation incident in Sudan in two months. At least 30 people were killed when a passenger plane caught fire at Khartoum airport in early June.

In May southern Sudan's minister of defence and a presidential adviser were among at least 23 people killed in a plane crash in the south. On Friday, a Sudanese cargo plane crashed in bad weather, killing seven of the eight crew. - Reuters