Pro-government forces have launched a major offensive from the Somali capital, Mogadishu, to seize territory from al-Shabab Islamist militants.
Some 1,000 soldiers backed up by 20 tanks captured three al-Shabab bases, a senior security official said.
African Union forces backing the government say they have advanced outside the capital for the first time.
Al-Shabab is under attack on several fronts, with troops from Kenya and Ethiopia also gaining ground recently.
Correspondents says this is the biggest joint offensive by the government and the AU force, Amisom, since August 2011.
Troops from Djibouti have recently arrived in Mogadishu to bolster Amisom's 12,000 soldiers, while the AU is asking the UN to approve a further 50% increase in troop numbers.
Al-Shabab controls many southern and central areas of the country.
The al-Qaeda linked group made a "tactical withdrawal" from most of the capital last year but has continued to stage suicide attacks in the city.
It confirmed that the pro-government forces had gained territory but vowed to hit back.
Six months of famine
A photographer with AFP news agency said he had seen the bodies of three government soldiers and an official after a convoy was hit by a rocket-propelled grenade fired by al-Shabab fighters.
"It was chaotic, most of the trees along the road had been felled by mortar fire and houses were damaged," he said.
Witnesses say several al-Shabab fighters died but neither side has confirmed any deaths.
Amisom spokesman Lt Col Paddy Ankunda said pro-government forces had seized Mogadishu University and Barakat cemetery as they advanced north from the city after "intense" fighting.
Hundreds of people have fled the clashes.
"This is the first time Amisom has been able to secure an area outside the parameters of the city allowing them to defend greater Mogadishu," said Lt Col Ankunda.
BBC East Africa correspondent Will Ross says the operation appears to be a concerted effort to clear the Islamist militants right away from the Somali capital.
But he says this conflict no longer has front lines and, with al-Shabab carrying out suicide bomb attacks, it will still be very difficult to make Mogadishu safe.
On Thursday, six people were killed by a suicide attack in a refugee camp.
The victims included a security guard and a local aid worker, witnesses said.
The bomb exploded just 20 minutes after a team of international journalists had left the Mogadishu camp.
They had gone to the city to see the situation six months after the famine was first declared in parts of the country, following the region's worst drought in 60 years.
Tens of thousands of people have died, aid workers say.
About 300,000 people have flooded into Mogadishu to seek food and shelter, as al-Shabab has banned most international aid agencies from areas they control.
The UN says a quarter of a million Somalis are still suffering.
On Thursday, medical charity MSF closed two of its health centres in the centres in response to the killing of two of its workers in the city last year.
Somalia has been wracked by two decades of conflict and lawlessness.