Syria is extraditing 21 suspected militants to Tunisia after recently handing over 12 others to Saudi Arabia, an official source said on Wednesday.

"We had 69 extremist suspects in custody. Of those we extradited 12 to Saudi Arabia and we are in the process of extraditing 21 to Tunisia," said the source, who asked not to be identified. "The rest are under investigation."

The 21 Tunisians were arrested in June and are linked to a Tunisian militant killed in a clash with Syrian security forces, the source said.

Damascus says the slain man had planned to set up a training camp for militants on Lebanese territory.

Syria said on Monday it had extradited 12 militants to Saudi Arabia, but the state news agency SANA gave no figure for the total number of Arab militants in Syrian custody.

Arab media have named the Tunisian killed on the border with Lebanon on June 22 as Majdi Bin Mohammad Bin Said Al Zreibi.

US and Iraqi officials have accused Damascus of not doing enough to prevent Arab militants from crossing into Iraq.

Syria says it is doing its utmost to secure the border.