Pirates have attacked a U.S.-flagged oil platform supply vessel off the coast of Nigeria and kidnapped two Americans, Fox News reported.

The attack on the C-Retriever ship, which is owned by Louisiana-based Edison Chouest Offshore, happened early Wednesday, UK-based security firm AKE and two security sources told Reuters. The ship is around 200-feet long and was sailing near Brass, Nigeria, in the Gulf of Guinea.

"Things are definitely getting more intense here," a source working offshore Nigeria told the maritime industry news website gCaptain. "This is the second attack on a security boat in the past three days."

The captain and the chief engineer of the C-Retriever were reportedly the ones onboard who were kidnapped.

A U.S. defense official told Reuters the State Department and FBI were leading the American response to the incident. A second defense official said the U.S. Marine Corps has a small training unit in the region but it was not clear if it would get involved.

The State Department is "closely monitoring" reports of the incident, Marie Harf, deputy spokesperson, told Fox News.

"Obviously our concern is their safe return," she said. "At this point we do not have information that would indicate this was an act of terrorism."

A law enforcement official also told Fox News the FBI believes it could be ransom case. No ransom demands have been made and no rescue attempt is imminent, according the NBC News.

U.S. Navy officials have grown increasingly concerned about piracy and armed robbery in the Gulf of Guinea and are working with local authorities in the area to strengthen their ability to patrol the region and better share information.

According to Reuters, Navy Secretary Ray Mabus called the region a potential "hot spot" after a visit to four countries surrounding the gulf in August.

He told Defense News in September the Navy was working closely with Gabon, Senegal, Sao Tome and Ghana to help fight an increase in illegal trafficking of drugs, people and arms.

"The piracy threat is spreading even further through the waters of West Africa, and the attacks have been mounting, even as global rates of reported piracy are at their lowest since 2006," said Michael Frodl of U.S.-based consultancy C-Level Maritime Risks.

Kidnapping for ransom has been a growing issue in Nigeria for several years. In 2006, ABC News interviewed an American, Texas Richards, who kidnapped by Nigerians and freed.

In 2009, on the other side of Africa, American Capt. Richard Phillips was held hostage on the Indian Ocean by a group of Somali pirates for five days before he was freed in a daring rescue by U.S. Navy SEALs. Hollywood recreated that ordeal in a recent blockbuster film starring Tom Hanks as Capt. Phillips.