The Syrian Human Rights Organisation said in a statement on Saturday the five had been detained during the past four days.
It identified two of them as Tahir Ali Mazraa and Shakir al-Khanji and said the other three were university students.
The rights group expressed fear Syria would hand over the detainees, suspected by authorities of having links with an opposition group in the southern Iranian province of Ahwaz, to Iran.
It called on the UN High Commissioner for Refugees to intervene to prevent their deportation.
The five - allegedly affiliated to the opposition Popular Democratic Front for the Arab Ahwazi People - may face imminent danger to their lives if handed over to Iran, the group said.
Fayiz Sara, a Syrian journalist, speaking to Aljazeera, said the number of Ahwaz Arabs arrested in Syria might rise.
"We fear the handing over of these prisoners to Iran, in particular because Syria has good relations with Iran," Sara said.
"Handing over the prisoners to Iran by Syria would violate international law"
"Handing over the prisoners to Iran by Syria would violate international law."
As per international law, no country is allowed to hand over prisoners to their country of origin if it endangers their lives, said Sara.
Syria and Iran are close allies on Middle East issues. Syria, until now, has not bothered the opposition group.
On 15 April, Iranian authorities arrested more than 250 Ahwaz residents after demonstrations to protest against alleged government plans to alter the predominantly ethnic Arab composition of Ahwaz city, the provincial capital, by relocating non-Arabs there.