African Union and Somali troops captured the key port of Marka from Al-Qaeda-linked Shabab insurgents on Monday, the latest in a string of bases to be wrested from the extremists, officials said.

"We have taken Marka, we entered alongside the Somali government forces this morning," said Colonel Ali Houmed, the spokesman for the African Union mission in Somalia (AMISOM).

"There was some fighting, but not so heavy, most of the Shabab had fled."

The loss of Marka, some 70 kilometres (45 miles) south of the capital Mogadishu, is another major blow for the insurgents, who have been on the back foot for several months.

AU and Somali troops have made significant gains in recent months against the Shabab, although the Islamists remain a major security threat. Ethiopian troops are also battling the militants from the south and west.

The loss of Marka leaves the Shabab with two major ports in southern Somalia -- Barawe and the key rebel bastion of Kismayo -- although an international naval blockade has already greatly squeezed maritime access there.

The Shabab abandoned their last fixed bases in Mogadishu a year ago, where they have since reverted to guerrilla tactics, claiming a series of suicide attacks and roadside bombs.

The latest defeat for the Shabab comes as candidates for Somalia's powerful position of parliamentary speaker campaign a day ahead of the expected vote, a key step in setting up a new government for the war-torn nation.

Somalia has not had a stable central government since the 1991 ouster of dictator Mohamed Siad Barre, which sparked rounds of bloody civil war.

The United Nations-backed process, which has already selected the majority of a new parliament and will culminate in a vote for president, is the latest bid to end two decades of instability in the Horn of Africa nation.

"We are facilitating the timely implementation of this important political step," the UN political office for Somalia (UNPOS) said Monday, adding it was hopeful the already-delayed vote would go ahead on Tuesday as planned.

Secret ballots in parliament for the posts of speaker and two deputy speakers have been delayed several times.

Bitter arguments have begun between challengers for the top jobs, divided along Somalia's notoriously fractious clan lines.

Six candidates are running for the post of speaker, including two former prime ministers, Hassan Abshir Farah and Ali Khalif Galayr, both from the Darod clan and the northern semi-autonomous Puntland region.

Mohamed Osman Jawari, a former minister under the regime of Siad Barre, from the Rahanweyn clan from the southern Baidoa region, is also running for the post.

The selection of speaker will impact the subsequent parliamentary vote for president, as Somali politics have traditionally tried to share out the seats between rival clans.

Around 260 of the new parliament's 275 members have been selected by a group of 135 traditional elders under the UN-backed process. Most were sworn into office last week on the tarmac of the capital's airport, protected by AU troops.