Thirteen people were killed and 20 others wounded in central Somalia on Saturday after a suicide bomber drove a truck packed with explosives towards a security checkpoint in the town of Beledweyne, police said.

"We have recovered the dead bodies of 13 people, most of them civilians who stayed nearby," Ahmed Yare Adan, a local police officer, told AFP by phone.

"Around 20 wounded people were already taken to hospitals, and we believe the number of the casualties could rise," he said.

The attack, which damaged nearby buildings, trapping people under the debris, came after Somalia's beleaguered government admitted to suffering "several significant setbacks" in its fight against Al-Shabaab militants.

The Islamist militants have waged an insurgency for over 15 years to overthrow the fragile internationally-backed government in Mogadishu.

An African Union force deployed in Somalia in 2007 with a six-month mandate but still remains on the ground, with the government now seeking to delay a planned reduction of foreign troops by three months.

UN resolutions call for the African Union force to be reduced to zero by the end of next year, handing over security to the Somali army and police.

Somali troops launched a major offensive against the Al-Qaeda-affiliated Al-Shabaab in central Somalia in August last year, joining forces with local clan militias in an operation backed by the AU force and US air strikes.

Somali President Hassan Sheikh Mohamud took office in May last year vowing "all-out war" against Al-Shabaab, who were driven from Mogadishu in 2011 but control swathes of the countryside.

Mohamud, who has recently been visiting the frontline, said in August that the government would "eliminate" the jihadists by the end of the year.

But Somalia's national security adviser wrote to the United Nations requesting a 90-day delay to the planned pullout of 3,000 AU troops by the end of September.

In the letter seen by AFP, he said the government had "managed to re-liberate towns, villages and critical supply routes" during its offensive but had suffered "several significant setbacks" since late August.