CAIRO -- At least six people were killed and nine others injured on Thursday when a tourist submarine sank in Egypt's Red Sea during a trip in the resort city of Hurghada, an official source at the Red Sea Governorate told Xinhua.

Five of the injured are in critical condition, while 29 other passengers were rescued and returned to their hotels, according to the source, who spoke on condition of anonymity.

He added that the cause of the incident is still under investigation, and the victims' nationalities have not yet been identified.

According to some local media outlets, including the widely read Al-Masry Al-Youm website, the tourist submarine, named Sindbad, was carrying 44 passengers on an underwater trip to explore coral reefs.

Meanwhile, RIA Novosti quoted a statement from the Russian Consulate General in Hurghada revealing that all the tourists on board were Russians.

"There were 45 tourists on board, including minors, as well as crew members. All of them were Russian citizens, tourists of the company Biblio Globus," the consulate's statement read, according to RIA Novosti.