Unknown assailants killed five nuclear engineers Sunday while they were on a bus just north of Damascus, near the research centre where they worked, a monitor said on Sunday.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of sources on the ground for its reports, later told AFP four of the engineers were Syrian, while one was Iranian.

"Unidentified attackers murdered five nuclear energy engineers who worked in the scientific research centre near the neighbourhood of Barzeh, northern Damascus," said Observatory director Rami Abdel Rahman.

He later said: "Four of the engineers were Syrian, and one was Iranian. Their bus was ambushed while they were on their way to the research centre. Their assailants shot them dead."

In July last year, six people who worked at the same centre were killed in a mortar attack carried out by rebels seeking President Bashar al-Assad's ouster.

Another military research centre, also near Damascus, was also hit by a deadly Israeli raid in May 2013.