Pirates have stormed an oil supply vessel off southern Nigeria and kidnapped two American crew members, Nigeria’s navy and US defence officials say, the latest unrest in a region described as a piracy hotspot.

A US-flagged C-Retriever owned by American oil servicing company Edison Chouest Offshore was attacked on Wednesday off the city of Brass, Nigeria’s navy spokesman Kabir Aliyu told AFP.

The ship’s chief engineer and captain, both US citizens, were kidnapped in the attack, said US defence officials who requested anonymity and AKE, a private security firm which closely tracks the region.

The US Navy and the Marine Corps had not yet received orders to intervene, two US defence officials told AFP.

Aliyu said the navy had launched a search and rescue mission, but could not discuss the details including whether any foreign navies were yet involved.

In Washington, State Department spokeswoman Marie Harf said on Thursday security officials at US missions in Nigeria were looking into “safely resolving the situation”.

“Obviously our concern at this point is for the safe return of the two US citizens,” she told reporters.

Edison Chouest Offshore did not reply to emails or calls seeking comment.

This was the first reported kidnapping of US nationals around the oil-producing Niger Delta region in at least two years, said an official at the AKE office in Lagos, who requested anonymity.

Abductions declined dramatically after a 2009 amnesty deal with rebels in the region, but they have spiked again in recent months.

Harf said that the United States was alarmed by the rising number of pirate attacks in the area.

“We are concerned by this increase. We’ve worked and will continue to work with states on the Gulf of Guinea to help them respond effectively to maritime crime in these waters,” she said.

On Tuesday, pirates shot dead two Nigerian officers who were escorting oil workers to a field off Bayelsa state, the same area where the Americans were abducted.