The Odisk, registered in Sierra Leone, was carrying about 2,000 metric tons of ferromanganese silicon from Ukraine to Turkey, Ukraine's Emergency Situations Ministry said.
A chemical reaction is believed to have started after rainwater got in touch with the alloy, used in metallurgy, in the Ukrainian port of Kerch during shipment, which provoked a poisonous gas to appear and kill a Russian and a Ukrainian.
The ship captain asked for help Friday. He said the crewmembers were poisoned by an unknown substance.
Emergencies services said there was no danger to the population after the ship was anchored not far from the coast.
An investigation is underway.