At least 30 members of Somalia's Al-Qaeda-affiliated Shebab militants were killed in a US drone strike on Thursday, the Kenyan government said, while backtracking on its earlier claim that the alleged mastermind of the Garissa University massacre was among the dead.

"Over 30 were killed, among them most wanted terrorists," Kenyan interior ministry spokesman Mwenda Njoka said.

He said that "several masterminds" of a string of recent attacks inside Kenya were killed, but retracted his earlier statement that the mastermind of the April Garissa University massacre -- Mohamed Mohamud, also known by the aliases of Dulyadin, Kuno and Gamadhere -- was among the dead.

"It was a US drone. Kenyan forces usually provide ground support, information and intelligence on such strikes," the spokesman said.

A statement from Kenya's defence ministry, meanwhile, said 51 Shebab militants were killed in the area -- although it said this was due to an "artillery bombardment" by African Union forces and made no mention of any drones.

"Fifty-one Al-Shebab militants were killed including several middle-level commanders who have been responsible for attacks in the Mandera region" of northeastern Kenya, the statement said, describing it as "major setback to the militants."

In April, four Shebab militants massacred 148 people at the Garissa University in Kenya's northeast, in what was the group's deadliest single attack to date. Most of the victims were students.

They have also conducted a string of attacks and massacres across northeastern Kenya, from Mandera in the far north to the Lamu region on the coast.

The United States has in recent years launched numerous drone strikes against Shebab leaders, including a strike last September that killed the group's leader Ahmed Abdi Godane.

Thursday's reported drone attack came just over a week before US President Barack Obama is due in Kenya for his first visit to the country since he became president.

Sources in Somalia confirmed an overnight air strike had taken place in a Shebab area of the war-torn country.

According to traditional elders near Bardhere town in the southern Gedo region, at least two missiles struck vehicles believed to be carrying Shebab commanders.

"We heard two big explosions and the information we are getting indicates that vehicles were targeted close to a Shebab military base," said Abdiwahab Ali, an elder at a village close to the scene.

"Village residents are telling us a missile fired from an aircraft struck a vehicle and a nearby military camp belonging to Shebab," said Hassan Gesle, another elder.

Immediately after the attack the mobile phone network in Bardhere was cut off, making it impossible to reach Shebab commanders for comment.

Ahmed Bare, Somali military officer in nearby Elwaq town, said that Shebab commanders have been leaving Bardhere, one of the few towns still held by the militants, ahead of a planned ground assault by Somali troops.