A suicide car bomber killed a district governor, his two sons and two bodyguards in Afghanistan's southern Kandahar province on Thursday, officials said, an area in which NATO-led forces have claimed security improvements over the past year.

The blast targeted the governor of Panjwai district, Sayed Fazluddin Agha, as he was travelling home from Kandahar city. He was on a road recently paved by Canadian forces, a route seen as important to undermining insurgent control of the area.

Nine policemen and a civilian were also wounded, said Zalmai Ayobi, spokesman for the Kandahar governor.

The attack came a day after a teenage suicide bomber managed to slip through tight security into Kandahar's police headquarters in an attempt to assassinate the police chief. He blew himself up without killing anyone else.

Both incidents raise serious concerns about security in an area that was the birthplace of the Taliban and saw fierce fighting during a surge of US troops over recent years.

The extra troops broke the insurgents' tight grip on much of the province, but there have been questions about how sustainable their gains will prove.

Despite the presence of more than 100,000 foreign troops, violence across Afghanistan remains at its worst levels since the Taliban were toppled by US-backed Afghan forces in late 2001, according to the United Nations.

Foreign forces fighting Taliban-led insurgents in Afghanistan are in the process of handing control of security over to the Afghan army and police, with foreign combat troops due to leave the country by the end of 2014.