The attackers captured five Polish nationals, including the captain and three officers of the small cargo ship "Szafir," Polish Foreign Minister Witold Waszczykowski said on Friday.

The assailants opened fire during the overnight incident in Nigerian waters. The ship sustained damage, but there were no reports of injuries and no traces of blood to be found, officials say.

The remaining 11 sailors managed to barricade themselves inside the ship during the encounter. They are safe and in contact by phone, according to Waszczykowski.

The incident appears to be a pirate attack, according to Sea Industry and Navigation Minister Marek Grobarczyk.

Piracy on the rebound

The ship was attacked while transporting metal cranes and other items from Antwerp in Belgium to Nigeria. "Szafir" is owned by a Polish company, but was sailing under a Cypriot flag.

The suspected pirates kidnapped the sailors some 30 nautical miles (56 kilometers) off the coast.

No official ransom demand was immediately made. Warsaw's government is in contact with Nigerian officials and waiting to open negotiations, Polish officials said. However, the Polish authorities would not get involved directly unless asked to do so.

"This is a responsibility of the sovereign state of Nigeria," Waszczykowski said.

Piracy has taken off in West Africa during recent years, with criminal groups hijacking ships to steal cargo, or taking hostages for ransom.

Earlier this year, suspected pirates killed four soldiers and a policeman in a coordinated attack on a military base in Nigeria's oil-rich Niger Delta.

dj/msh (AFP, AP, dpa, Reuters)