Engine failure has been blamed for a plane crash in which South Sudan's defence minister was killed along with at least 22 other people.

"Two engines failed and there was nothing the pilot could do," said Salva Kiir, President of South Sudan.

Mr Kiir announced three days of mourning in southern Sudan.

Defence Minister Dominic Dim Deng had been flying with a military delegation to the regional capital, Juba. A presidential adviser was also killed.

The UN, which sent a helicopter to the site of the crash, said the plane was a Beechcraft 1900 operated by South Sudan Air Connection that had been travelling from Wau to Juba.

Officials said an investigation into the crash would be opened.

Philip Yona Jambi, South Sudan's rural development minister, said that the pilot asked permission to make a crash landing in Rumbek after reporting engine trouble.

He said passengers on board included Mr Deng and his wife, senior presidential adviser Justin Yak and his wife, and other senior military officials.

His widow called his death an assassination, despite an official probe that blamed pilot error.

Mr Garang died shortly after a 2005 peace deal, which ended decades of civil war between the ethnic African south and the Arab-dominated government in Khartoum.

Mr Jambi said that the news had shocked many southern Sudanese.

"People are very worried and those who have heard about it will suspect foul play because John Garang died in a crash and they still don't believe the results of the investigation," he said.