A suicide bomber on a motorcycle has killed the deputy governor of Afghanistan's southern Kandahar province, a NATO official and the governor's spokesman has said.

Abdul Latif Ashna was killed inside Kandahar city, capital of Kandahar province, Zalmay Ayoubi, the governor's spokesman said on Saturday.

Four others were injured in the blast. Three of those were the bodyguards of the governor, who was reportedly attacked while on his way to work.

Thousands of US-led forces have stepped up operations against insurgents fighters in and around the city over the past year.

US ambassador, Karl Eikenberry, who was visiting Kandahar on Saturday condemned the death but said it would not impede efforts to defeat the insurgency.

"The loss of a great deputy governor like this is a setback. What we've seen is consistently Afghan government leaders emerge and the people continue to rally in an effort to establish security in this province," he said.

A war review by Barack Obama, the US president, last month said "notable operational gains" had been made and the Taliban's momentum stopped in much of the country.

But critics say that statistics show insurgent attacks are at their highest since the war started.

Fighters have also stepped up the use of targeted assassinations, particularly government and political figures over the past year.

Between mid-June and mid-September, 21 people were reported to be assassinated each week, the United Nations has said.