An explosive device was thrown at a convoy carrying the head of the United Nations mission to Libya on Tuesday, a UN spokeswoman said, adding that there were no casualties.

"While on a visit to Benghazi, the head of the United Nations Support Mission to Libya had what appeared to be an explosive device thrown at his convoy," Hua Jiang said.

UN envoy Ian Martin, who is tasked with assisting the interim authorities in their transition to democracy, was aboard the convoy.

No one was hurt in the blast, which could have been caused by a hand grenade, Hua told AFP.

Mohammed al-Gezziri, spokesman of the high security council of the city, confirmed there were no casualties and said "investigations were underway to determine who is behind the attack."

Gezziri said the convoy was attacked on its way to visit his council.

He said investigators will use footage recorded by security cameras in a bank near the site of the blast to identify the assailant.

"The person who carried out the attack wanted to send a message to the West that Libya is not stable when the opposite is true," he said.

Several Western and Arab diplomats have visited the country without incident since the regime of Moammar Gaddafi was toppled last year.

But the blast could push foreign dignitaries and expatriates in the oil sector to think twice before coming back to the north African nation.

Libya's neighbours and analysts warn that Gaddafi's weapons have spread across the region and fallen into the hands of Al-Qaeda linked groups.