A car bomb has exploded at the gates of Somalia's presidential palace, followed by an attack by gunmen, police in Mogadishu say.

Some reports say there is now a gunfight inside the building.

President Hassan Sheikh Mohamud has called the UN envoy to Somalia to say he was not harmed during the attack, envoy Nick Kay has tweeted.

The UN-backed government is battling al-Qaeda-linked group al-Shabab for control of the country.