Three Iranian seamen have died after a toxic substance poisoned all 11 crew members of a cargo ship on the Caspian Sea, according to Iran's official IRNA news agency.

Crew members contacted authorities in Azerbaijan, its destination, as they were approaching the country and requested help.

An Azerbaijani coast guard boat delivered the ill members of the crew to the shore early on Friday.

The crew was taken to hospital in Baku, the capital, but three sailors died on the way to the facility.

The ship's captain stayed aboard the vessel, named Nazmehr, and was given medical help, Azerbaijan's health care ministry said.

IRNA said that the poisoning was under investigation.

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The vessel was carrying grain from Aktau, Kazakhstan.

Ministry spokesman Parviz Abubekirov said preliminary evidence indicated that the crew had been poisoned by chemicals used to protect the grain.