Somali and African Union forces have launched a fresh offensive to roust Shebab Islamist fighters from key ports, army and government officials said Saturday, in an effort to cut off a main revenue source for the extremists.

"Operation Indian Ocean started late last night.... The enemy is fleeing and the forces are making successful advances so far," said Abdukadir Mohamed Nur, the governor of southern Somalia's Lower Shabelle region.

Witnesses reported hearing booms from heavy shelling and seeing convoys of tanks and armoured vehicles heading towards the Shebab stronghold of Bulomarer, some 160 kilometres (100 miles) southwest of the capital Mogadishu.

The Al-Qaeda-linked Shebab are still a powerful and dangerous force but have lost a string of towns to the 22,000-strong AU mission in Somalia (AMISOM).

Deployed in Somalia in 2007, AMISOM has made a series of advances in the past three years, but the launch Saturday of a new offensive targets the Shebab's lucrative charcoal trade.

AMISOM and Somali government troops were also seen on roads towards Barawe, the last major port held by the hardline gunmen on Somalia's Indian Ocean coast.

"We are heading towards Bulomarer town," Nur added. "The operations will not stop until the Al-Qaeda militants are eliminated."

In January 2013, French commandos attempted a raid on Bulomarer in a bid to free a secret agent held hostage.

The bid failed and resulted in the death of two French soldiers and the hostage.

AMISOM said it had advanced and captured a small settlement of Golweyn, on the road south towards Barawe.

In previous offensives by AU troops, Shebab fighters have fled in advance of the main column but later returned to stage guerrilla-style attacks.

Shebab fighters continue to launch attacks even in the heart of Mogadishu, including recent brazen commando raids on the presidential palace and parliament.

- 'Appalling' hunger and drought -

"We are hearing heavy shelling," said Ali Mohamed, who lives close to the Bulomarer, adding that fighting close to the town.

"I saw an AMISOM military convoy including several tanks this morning headed towards Bulomarer and Barawe," said Hussein Mumin, another local resident.

The southern port of Barawe is now one of the few major settlements under their control, and is vital to Shebab finances, as the main hub of a multi-million dollar charcoal trade.

Charcoal, which is mainly exported to Gulf nations, generates at least $25 million (19 million euros) a year for the Shebab, according to UN estimates.

The Shebab are fighting to topple Somalia's internationally-backed government, and regularly launch attacks against state targets, as well as in neighbouring countries that contribute to the AU force.

The fighting comes as UN and aid workers warn large areas of Somalia are struggling with dire hunger and drought, three years after famine killed more than a quarter of a million people.

Somalia's government, selected in a UN-backed process in 2012, was widely hailed as offering the best chance in decades to repair the war-ravaged country.

But the return of extreme hunger, accusations of corruption and continued Shebab attacks in even the most heavily defended zones have cast a shadow over the government's record.

On Saturday Minister of Information Mustafa Duhulow said assessments in the southern Bakol and Gedo regions had found "appalling" conditions.

"Both areas are in urgent need of humanitarian assistance," he said.