Gunmen killed five nuclear engineers, four of them Syrian and one Iranian, on the outskirts of Damascus on Sunday, a monitoring group said on Monday, Reuters reported.
No one claimed responsibility and Syrian and Iranian state media did not mention the attack, which occurred in an area controlled by forces loyal to President Bashar al-Assad.
It was not clear in what capacity the Iranian nuclear engineer was in Syria, if his presence were to be confirmed.
The Syrian Observatory for Human Rights said the engineers were shot dead while travelling in a small convoy to a research centre near the northeastern district of Barzeh.
Iran has supported Assad throughout Syria's three-year civil war and Iranian military advisers are working with Syrian forces throughout the country, which is partly under insurgent control.
Both Iran and Syria are under investigation by the U.N. nuclear watchdog, the IAEA. Both countries have repeatedly denied ever having any nuclear weapons ambitions.
The IAEA said last year that Syria declared a "small amount of nuclear material" at a Miniature Neutron Source Reactor (MSNR), a type of research reactor usually fuelled by highly enriched uranium, near Damascus.
Former IAEA chief inspector Olli Heinonen said there are two nuclear centres on the outskirts of Damascus and that the MNSR is one of them. Heinonen, now at Harvard University's Belfer Center, said that the centres do not appear to have had extensive nuclear fuel cycle activities.
The IAEA used to visit the MNSR about once a year to check nuclear material there but cancelled such inspections last year due to the violence in the country. The MNSR is believed to hold less than a kilogram of highly-enriched uranium, well below the roughly 25 kg which experts say would be needed for any bomb.
IAEA inspectors in mid-2008 examined a site in the eastern province Deir al-Zor that U.S. intelligence reports say was a nascent, North Korean-designed reactor geared to making plutonium for nuclear bombs. Israel bombed it in 2007.
But Syrian authorities later repeatedly rebuffed IAEA requests to revisit the site to gather evidence, saying it had been a conventional military base only. The Deir al-Zor region is now largely under the control of Islamist insurgents.
Iran, the United States and European Union began a second day of talks in Oman on Monday to discuss ways to resolve a confrontation over Tehran's nuclear programme.