Kenyan fighter jets have bombarded an airport in southern Somalia, where they are fighting al-Qaeda linked al-Shabab fighters, officials have said.

The strikes took place in the port city of Kismayo on Tuesday.

"Our forces have reached Kismayo with jets and they have destroyed the armoury and a warehouse used by al-Shabab at the airport," Cyrus Oguna, a Kenyan army spokesperson, said.

He could not provide figures on the number of casualties incurred.

Ali Mohamud Rage, an al-Shabab spokesperson, played down the impact of the bombing raid and said that it had not resulted in any deaths.

"No one was killed and there was no property damage," he told the AFP news agency. "The Kenyan airforce was maybe trying to boost the morale of its demoralised soldiers."

'Heavy explosions'

Residents reported at least three heavy blasts near the airport, where the fighters are based.

"The explosions were very heavy and they rocked the airport," said Abdi Ugas, a witness.

"They targeted the airport... one of them was very heavy," said Osman Ali, another Kismayo resident.

The city is the last major bastion of al-Shabab, who have lost most of their other strongholds to the 17,000-strong African Union force - of which Kenya is a part - as well as allied Ethiopian forces.

Kenyan troops have been aiming to defeat al-Shabab in Kismayo ever since they were deployed across the Kenya-Somalia border almost a year ago.

Kenyan soldiers are still about 40km from Kismayo.