Thousands are fleeing Afmadow as Kenyan soldiers close in on the town [Reuters]

A military jet has targeted the southern Somali port of Kismayo, an al-Shabab stronghold, as Kenyan and Somali government forces prepare to attack another town held by the rebel group.

A Kismayo resident told the Reuters news agency on Monday that the jet had bombed an al-Shabab base near the port.

"It dropped a huge shell, flew past, came and then dropped another shell," said Mahmoud Hassan.

An al-Shabab official who declined to be named also confirmed the news. "There were no casualties. We fired at the plane after the second bombardment and it has not come back," the senior official told Reuters.

However, the casualty figure has yet to be corroborated by independent sources, Reuters said.

Although the identity of the jet has not been confirmed, Kenyan troops began an air and ground offensive against al-Shabab in southern Somalia last week, blaming the group for several kidnappings of foreigners on its soil, including a French woman who later died in captivity.

A Kenyan military spokesman on Sunday said that a French navy ship had bombarded the town of Kuday, near Kismayo on Saturday night, but French officials have denied their forces were involved in any attacks.

The French embassy in the Kenyan capital Nairobi issued a statement on Monday refuting claims that French navy vessels carried out bombardments, arguing that "there are no French warships in the vicinity of this area" and that the "embassy would appreciate if the newspapers would kindly correct the false impression created on this matter". 

Controlled by pirates 

Al Jazeera's correspondent Peter Greste, based in Nairobi, said, "What we do know is that the Kenyan military spokesperson said the attack did take place on the town of Kuday, which we understand is under the control of pirates rather than al-Shabab."

The Kenyan military spokesman also said fighting was likely to occur in the town of Afmadow, near Kismayo, "very soon".

Hundreds of residents were fleeing Afmadow on Sunday in anticipation of the fighting. The spokesman, Maj. Emmanuel Chirchir, told AP that al-Shabab was regrouping in the town of Bula Haji to face the Kenyan troops.

Somali interim government forces are also joining the offensive against al-Shabab, backed by 9,000 African Union peacekeepers from Burundi and Uganda.

Al-Shabab retreated from Mogadishu amid a devastating famine a few months back, but have re-emerged by staging their deadliest single bombing in the capital earlier this month, killing more than 100 people.

The AU mission to Somalia force, also known as AMISOM, said in a statement on Sunday they had advanced to Mogadishu's outskirts.