Mogadishu's main market was also bombed on Tuesday
Islamist insurgents in Somalia say they have taken control of the southern port of Kismayo amid fighting that has left dozens of people dead.
A spokesman for al-Shabab, Mukhtar Robow, told the BBC his militia had wrested the city from a local clan militia during a third day of clashes.
A UN official said about 100 people had been killed and up to 25,000 displaced.
There has also been fierce fighting in the capital, Mogadishu, and hijackings by pirates off the north Somali coast.
Al-Shabab is a radical wing of the Union of Islamic Courts, which ruled much of Somalia in 2006 before being ousted and launching a rebellion.
The BBC's Mohamed Olad Hassan says Kismayo, Somalia's third city, is strategically important because it serves as a port for the south of the country.
On Friday at least 15 people were reported to have died in the Kismayo fighting and 18 injured, with dozens killed over three days of clashes.
Mark Bowden, the UN humanitarian co-ordinator for Somalia, told the BBC's Focus on Africa programme that about 100 people had been killed in Kismayo and as many as 25,000 displaced.
"After four hours [the fighting] ended up in the northern corner of the town, now the town seems to be under the control of al-Shabab," a human rights worker in the port told the BBC on Friday.
Residents said Islamist fighters were patrolling the streets, and that sporadic shooting was continuing in parts of the city.
The fighting is said to have caused an acute humanitarian crisis.
Many people have no access to food and all business activity is reported to have stopped.
"The last three days of fighting has severely affected the town, where people remained in doors," one resident said.
"Now I am out, to my surprise, I saw a single wheelbarrow full of bread being mobbed by a crowd of people."
In Mogadishu on Thursday, some mortars landed near the compound of President Abdullahi Yusuf, who was out of the country.
Another landed near a mosque in the busy Bakara market, killing at least six people, a witness told the BBC.
Witnesses said government troops and their Ethiopian allies responded by opening fire, killing several civilians.
At least 20 people were reported to have been killed in fighting in the capital, though the city was calm on Friday.
Ethiopian troops entered Somalia in December 2006, to oust Islamist forces from Mogadishu.
The police chief in the capital said people who wanted to sabotage talks in neighbouring Djibouti between Somalia's provisional government and its Islamist rivals were behind the most recent violence, our correspondent reports.
Somalia has been without a functioning national government since 1991 and has suffered ongoing civil strife.
The UN's World Food Programme is expanding its programme to feed 2.4 million people in Somalia by the end of the year.
Aid efforts have been hampered by the violence, and the delivery of aid has been threatened by piracy near Somalia's coast.
On Friday, the International Maritime Bureau (IMB) said pirates had seized a German cargo ship off the Somali coast a day earlier.
Earlier, a Japanese tanker and an Iranian bulk carrier had been hijacked in the Gulf of Aden, a busy international shipping route to the north of Somalia.
An IMB spokesman said a warship from an international force was tracking the hijacked ships.
Another ship, a Malaysian oil tanker with 39 crew was captured in the same area on Tuesday.