UN Secretary General Kofi Annan expressed concern over recent unrest in Afghanistan, saying it underlined "deeper problems" that needed to be addressed by the international community.

UN Secretary General Kofi Annan expressed concern over recent unrest in Afghanistan, saying it underlined "deeper problems" that needed to be addressed by the international community.[AFP\File]

Annan said he spoke by telephone with Afghan President Hamid Karzai and US Secretary of State Condoleezza Rice "about measures we can all take to help bring the situation under control."

The UN chief's comments came after a traffic accident on Monday involving a US military truck set off deadly riots across the Afghan capital Kabul.

"I have followed with concern the developments in Afghanistan," Annan told reporters at UN headquarters after returning from a two week tour of Asia.

"Obviously what has happened is symptomatic of perhaps deeper problems and the need we have to work with them to strengthen their security forces," he said, adding that the Kabul unrest followed "major disturbances" in other regions of the country.

"We have an insecure situation in parts of the country, we have the question of drug cultivation and production, we have the need to strengthen security forces and national institutions," he said.

Solving the country's problems with opium cultivation and weak central institutions would "take time and resources but we need to persevere," Annan said.

Police in Afghanistan said 12 people were killed and more than a 100 were injured in riots that erupted after a US truck ploughed into several cars. A mob gathered at the accident scene and started pelting the US soldiers, who then opened fire.

The US-led coalition said the soldiers shot above the crowd while witnesses said they saw at least four people die in the gunfire.

Buildings, police posts and cars were torched as well as offices of international aid groups. Kabul was calm on Tuesday after a six-hour overnight curfew though heavily-armed police remained on alert.