(Updates the death toll)

MOGADISHU Aug 21 (Reuters) - More than 20 people were killed on Sunday when suicide bombers from the militant al Shabaab group detonated two car bombs at a local government headquarters in Somalia's semi-autonomous Puntland region, witnesses and officials said.

Residents of the town of Galkayo in north central Somalia said they heard two loud blasts in quick succession followed by heavy gunfire.

"There were two huge bombs. The first one was a truck bomb, followed a minute or so (later) by another car bomb. My brother was injured at the scene," Halima Ismail, a local resident, told Reuters.

Islamist al Shabaab, which has carried out a series of deadly attacks in the Horn of Africa country as it seeks to topple the Western-backed government, claimed responsibility for the bombings.

"There were two suicide car bombs," Abdiasis Abu Musab, the group's spokesman for military operations, told Reuters.

Police initially put the death toll at 10, including civilians and security forces personnel, but a medical official said that number had doubled.

"Death toll is over 20 and it may rise further," said Ahmed Sugule, a doctor at Galkayo hospital. He said another 30 people were wounded in the attack.

Al Shabaab has become more active in Puntland, a large part of northern Somalia, since moving more forces there after being pushed out of strongholds farther south by an African Union force and the Somali National Army, experts and officials say.

The group controlled Mogadishu for several years until 2011 when African Union forces drove it out. (Reporting by Abdiqani Hassan, Abdi Sheikh and Feisal Omar; Writing by Duncan Miriri; Editing by Helen Popper)