"The 21 arrested were detained in connection with the latest incidents in Syria, in particular those of June 22 during which a Tunisian extremist, who was planning to set up a camp to train extremists in Lebanon, was killed on the Syrian-Lebanese border," SANA said on Tuesday.
They were handed over "as part of cooperation with Tunisian authorities in the fight against terrorism", it said.
Damascus has previously said there were several clashes between security forces and "terrorist" groups in June and July, including a shootout with a number of deposed Iraqi leader Saddam Hussein's ex-bodyguards.
Damascus said on Monday that it had handed over another 12 alleged militants to Saudi authorities.
Syrian Deputy Foreign Minister Walid al-Muallem on Thursday said his country had detained 1240 foreign extremists trying to enter Iraq.
Al-Muallem did not say when the arrests happened or identify the nationalities of those arrested but did say that most of the foreigners were handed over to their embassies.
Baghdad and Washington have accused Damascus of turning a blind eye to fighters crossing into Iraq or of supporting the raging fighting, charges denied by Syria.