"Everything indicates this was by accident," a spokeswoman for North Carolina's Morehead City Police Department said.

The state Ports Authority said there were no reported injuries or damage to the environment.

A forklift operator accidentally punctured some of the drums as they were being unloaded onto a truck, a port authority official quoted emergency managers as saying.

They described the shipment as legal, licensed cargo from Spain.

The police spokeswoman said that as a precaution the port was shut down and evacuated. An evacuation recommendation was also issued for downtown Morehead City.

The port is one of the deepest on the U.S. East Coast. Its traffic includes imports of chemicals like sulfur and phosphates.

Morehead City Mayor Jerry Jones told CNN that emergency services had sealed off the port area where the explosive substance had spilled from drums onto the port property, but he said the accident was contained.

"I can confirm it is PETN (Pentaerythritol Tetranitrate) and it is explosive," he added.

Military explosives experts, police and fire officials were at the scene.

PETN is used as an industrial and military explosive and requires a federal license to be handled and used. The substance was part of the ingredients of the bomb involved in the failed Christmas Day attack against a Detroit-bound U.S. airliner. A Nigerian man was charged in that incident.

The local ports authority said explosive materials were handled routinely at Morehead City Port. It said the circumstances of the accident were being investigated.

The port is the second-largest U.S. natural rubber importer, behind the port of New Orleans.

People who remained in their homes in downtown Morehead City were being asked to stay away from windows and doors, according to local media