Somalia's government said it will bar ships from using rebel-controlled southern ports in an effort to starve the militants of revenue and supplies, but it was unclear how it would enforce the ban.

The U.N.-backed government acknowledged it would need international help to intercept merchant vessels heading to three ports controlled by al Shabaab militants south of the capital Mogadishu.

The move comes amid a new offensive by government troops backed by African Union peacekeepers to break the back of a four year insurgency that has killed at least 21,000 people.

With the help of the international community, the Somali government will take any boat or ship taking any kind of goods to al Shabaab. Kismayu, Barawe and Marka ports will be closed, the government said in a statement late on Tuesday.

The African Union last year urged the U.N. Security Council to back a naval blockade aimed at shutting off the flow of arms and recruits to the insurgents.

Many Somali merchants favour importing goods through ports like Kismayu rather than Mogadishu, citing rampant corruption in the capital and lower taxes and charges at provincial ports.

Fighting in Mogadishu also has made it difficult to access Bakara market, Somalia's largest and also one of the main rebel hideouts. The violence has driven up food and fuel prices.

Local businessmen doubted whether the government had the capability to implement the ban.

(If) they can implement it, it will be an unkind, quick decision, trader Barre Hussein told Reuters. There are many civilians who will suffer. Why doesn't the government end piracy, lower taxes and open access to Bakara?