Five Syrian nuclear scientists have reportedly been killed by unknown assailants while riding a bus north of Damascus, near the research center where they worked, the UK-based Syrian Observatory for Human Rights reports.

“Unidentified attackers murdered five nuclear energy engineers who worked in the scientific research center near the neighborhood of Barzeh, northern Damascus,” Observatory director Rami Abdel Rahman told AFP.

The news agency meanwhile reports that four of the atomic engineers were Syrian, while the other one was Iranian.

“Their bus was ambushed while they were on their way to the research centre. Their assailants shot them dead," Rahman said.

So far no one has claimed responsibility for the attack.

The UN International Atomic Energy Agency (IAEA) said last year that Syria had declared a “small amount of nuclear material,” most probably highly enriched uranium, used in a Miniature Neutron Source research reactor, near Damascus, Reuters reported.

This is not the first time that Syrian scientists have been targeted in deliberate assassinations.

Workers at the Scientific Research Center in Barzeh were targeted by a group of jihadists who shot an RPG at their commuter bus on July 31, 2013, killing 6 workers and injuring 19 others.

Another military research center was also hit by a deadly Israeli raid in May 2013.

According to Nuclear Threat Initiative (NTI), a nonpartisan, nonprofit organization, Syria is non-nuclear weapon state party to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) since 1969.

Damascus is currently known to possess only one small operational research reactor, the Chinese built SRR-1, used for research of peaceful energy needs. The Atomic Energy Commission of Syria (AECS) directs Syria's nuclear program, and the majority of Syria's nuclear-related work takes place at the Der Al-Hadjar Nuclear Research Cente and the Scientific Studies and Research Center (SSRC) in Damascus.