Its capture -- reportedly without a fight -- is a key step towards opening up the main highway linking the capital Mogadishu to Baidoa, a major town wrested from the Shebab by Ethiopian troops in February.

The AU mission in Somalia (AMISOM), which also seized the Balli Doogle airstrip on Sunday, have now advanced over 90 kilometres (55 miles) northwest from Mogadishu since launching an offensive outside the city in May.

"Somali government forces with the support of AMISOM troops have secured the strategic town of Wanla Weyn on the Afgoye-to-Baidoa corridor," said Andrew Gutti, commander of the 17,000-strong AMISOM force.

Wanla Weyn lies roughly a third of the way to Baidoa from Mogadishu, with the remaining 160 kilometres (100 miles) still controlled by the Shebab.

"The capture of this town denies Al Shebab another source of illegal income after a string of defeats," Gutti added in a statement.

"It will also facilitate the provision of much-needed humanitarian assistance to the local population."

Kenyan troops -- who invaded Somalia a year ago before later integrating into AMISOM -- are also pushing up from the south, having last week seized the Shebab bastion and major port of Kismayo.

The hardline insurgents still control the town of Jowhar, some 80 kilometres northeast of Mogadishu, and the small port town of Barawe, lying some 180 kilometres down the coast from the capital.

However, the fighters have largely retreated ahead of each assault, with some reportedly relocating to the northern Galgala mountains in Somalia's semi-autonomous Puntland region.

In addition, vast and remote areas of bush surround many of the towns seized, which could also provide a haven for fighters who have fled.

But the Shebab remain a potent threat, still controlling rural areas as well as carrying out guerrilla attacks -- including suicide bombings -- in areas apparently under government control.

The latest advance comes a day after Somalia's newly elected President Hassan Sheikh Mohamud named his prime minister, businessman Abdi Farah Shirdon Said, in the first major decision of the administration.

Mohamud, who took office last month after being chosen by parliament in a UN-backed process, is being supported by international efforts in the hope of rebuilding stability after more than two decades of anarchy and war.

However, Shebab spokesman Ali Mohamud Rage dismissed the new prime minister as "yet another apostate appointment by foreign powers."

The death toll rose from an initial 13 after two victims succumbed to their injuries, army spokesman Sawarmi Khaled Saad was quoted as saying by the state SUNA news agency.

It is the worst toll in a series of Sudanese military aviation incidents since early 2011 and follows a recent surge of unrest in Darfur, the far-west region where a rebellion began nearly a decade ago.

A military investigation team has visited the crash site and expects to conclude its probe soon, Saad said.

"The pilot informed the airport that he had a problem with one of his engines," before the plane went down west of Jebel Aulia, he told AFP earlier.

Debris was scattered across red desert sand charred black from the impact, a photograph taken at the scene showed.

Saad said the plane, with six crew and 16 other members of the armed forces, was carrying military equipment from Khartoum to El Fasher, the capital of North Darfur state.

A witness said a helicopter ferried the dead and injured from the crash scene, which had been sealed off by soldiers, about 70 kilometres (43 miles) west of the capital.

Jebel Aulia is a popular recreational site about one hour's drive south of the Khartoum area.

Sudan's armed forces, which rely on Russian-made planes and helicopters, have suffered a number of aircraft losses in recent years.

In July, Darfur rebels said they shot down an Mi17 helicopter, killing seven personnel from the military which blamed a malfunction for the incident.

Last December, all six crewmen aboard another military helicopter died when it crash-landed and burned in North Kordofan state. The army cited a technical problem, as it did in April 2011 when a helicopter went down in Darfur killing all five soldiers on board.

Washington on Thursday voiced concern that security in western Darfur was worsening and threatening the implementation of peace accords there.

"The United States is deeply concerned by the sharp deterioration in security in North Darfur and adjacent parts of Jebel Marra, Sudan," State Department spokeswoman Victoria Nuland said.

More than 70 civilians were killed in Hashaba, North Darfur, between September 25 and 27 in fighting and aerial bombardments between rebels and the Sudanese government forces, she said.

Washington was also "appalled" by Tuesday's attack on a UN patrol in which four Nigerian peacekeepers were killed and eight wounded, she added.

Rebels from black African tribes rose up against the Arab-dominated Khartoum government in 2003. Clashes with government troops, banditry and inter-ethnic fighting continue but levels of violence have fallen compared to nearly a decade ago.