Pirates attacked an American-flagged oil industry vessel off the Nigerian coast early Thursday and abducted the captain and the chief engineer, both United States citizens, the Nigerian Navy and a private security firm reported. The abductions appeared to be the first involving American hostages in that region in at least two years.

An official of the private security firm, AKE Group, of Hereford, England, said the attack on the vessel, identified as the C-Retriever, took place near the Nigerian city of Brass, where the oil-rich Niger Delta empties into the Gulf of Guinea, in West Africa. The official, based in AKE Group’s office in Lagos, Nigeria, spoke on the condition of anonymity.

“All we know is this attack happened, and these were the people who were kidnapped,” the official said by telephone. He said he did not know the identities of the two hostages.

A spokesman for the Nigerian Navy, Cmdr. Kabiru Aliyu, confirmed the piracy attack. “The Nigerian Navy has directed its operational command to search for and rescue the vessel and the crew members,” he said. “Right now, the search is going on, and we are tracking down the culprits. We don’t know how it was carried out.”