* At least 20 civilians killed on Wednesday
* Clashes also erupt on border with Kenya
MOGADISHU, Feb 23 (Reuters) - Somali troops backed by peacekeepers seized three rebel bases in the capital Mogadishu on Wednesday, the government said, in a new offensive against hardline Islamist militants.
The country's interim government -- whose mandate ends in August with no firm plans in place for what will follow -- has struggled to end a four-year insurgency that has killed at least 21,000 people.
From early morning, Burundian troops from the African Union (AU) peacekeeping force, known as AMISOM, advanced block by block on foot along the city's industrial road, which leads to the Bakara market, a major insurgent hideout.
"We have captured three al Shabaab bases," Defence Minister Abdihakim Haji Fiqi told reporters. "Our enemies have been weakened and the fighting will continue until we have captured the city. AMISOM says its soldiers have now secured more than half of Mogadishu covering more than three quarters of the population. It was not possible to reach al Shabaab for a comment. An al Shabaab suicide car bomb killed at least 17 people near a police training camp in Mogadishu on Monday. Prime Minister Mohamed Abdullahi Mohamed said there could be further bomb attacks by the rebels loyal to al Qaeda.
"We understand the weakened al Shabaab may plan bombings. But we are very vigilant and have brave forces," Mohamed told a news conference in the rubble-strewn capital, unusually dressed in military fatigues.
Ali Muse, coordinator of ambulance services, reported that at least 20 people had been killed. Residents said Wednesday's clashes were the heaviest since the latest bout of violence began on Saturday. "Stray bullets pour like rain," Muse said. Separately, a Kenyan official said security forces were on high alert along the border with Somalia because al Shabaab and government-allied militia were fighting in the Somali border town of Balad Hawa. East Africa's largest economy has long cast a wary eye at its anarchic neighbour where first clan warlords and now Islamist insurgents have reduced government to impotence.
"We are on alert and aware that rebel groups are fighting. They are fighting inside Somali territory," said David Ole Serian, provincial commissioner of the frontier North Eastern province. Kenya closed its border with Somalia in January 2007. However thousands of Somalis still cross the desert frontier each year to reach refugee camps in Kenya. "Families living close to the border have moved away with their belongings. People are scared. School children have been sent home and many Somalis have crossed the border," said primary school teacher Ibrahim in Mandera, a Kenyan town little more than a stone's throw from Balad Hawa.
(Additional reporting by Sahra Abdi in Nairbi and Noor Ali in Isiolo; Writing by Richard Lough; Editing Helen Nyambura-Mwaura and Elizabeth Piper)