Al-Shabaab movement on Friday has freed 400 prisoners in Somalia, while four people were killed during the operation.
The attack took place in the central prison in Bosaso in the state of Puntland, northern Somalia.
A local security official announced: "It is too early to determine the exact number of victims, but we confirm at the present time that four people were killed and several others were injured."
The central prison is still under siege, according to the same source.
Somali state TV confirmed the attack, reporting that: "Bosaso Central Prison is under attack, and the security forces are addressing the situation."
Bosaso Central Prison contains a number of prisoners, including those convicted of terrorism and having ties with Al-Shabaab movement.
Colonel Hussein Ali Mahmoud disclosed in a statement: "The Puntland forces managed to counter the attack launched by Al-Shabaab militants on the prison."
Colonel Mahmoud added: "The attack caused human fatalities, we will announce more details at a later time. A number of prisoners escaped, but some of them were arrested by the security forces."
Al-Shabaab movement have claimed responsibility for the attack through its Alameda website.
The website stated: "Al-Shabaab militants launched a surprise attack in several directions on Bosaso Prison, and were able to control the offensive."
The website added: "Al-Shabaab fighters stormed the prison during the attack and freed 400 prisoners, including members of the movement."
Bosaso is a commercial coastal city located 1,388 kilometres from the capital, Mogadishu.
For years, the Somali authorities have been waging a war against Al-Shabaab movement, which was established in early 2004. It is an armed movement ideologically affiliated with Al-Qaeda and has claimed many operations responsible for the loss of hundreds of lives.