Pirates have hijacked a Greek-owned bulk carrier off the Somali coast, the latest attack on commercial shipping in the region.
The capture of the MV Irene EM hours is the latest piracy incident despite two raids on pirates in recent days by US and French special forces.
NATO Lieutenant Commander Alexandre Fernandes said the Portuguese warship NRP Corte-Real had received a distress call from the St Vincent and the Grenadines-flagged merchant ship as it travelled through the Gulf of Aden.
Heavily armed gunmen from the failed Horn of Africa state have run amok through the busy shipping lanes of the Indian Ocean and strategic Gulf of Aden, capturing dozens of vessels and making off with millions of dollars in ransoms.
NATO officials said a Canadian warship had sent a helicopter to investigate what was happening.
Elsewhere, Somali pirates have opened fire on another cargo ship off the Horn of Africa.
NATO staff officer Stephan Gresmak said ten pirates on board three skiffs fired automatic rifles and rocket-propelled grenades at the Liberian-flagged 21,887-tonne Safmarine Asia.
There was no immediate word of any casualties.
Foreign navies are continuing to patrol the waters off Somalia, but many pirates have so far evaded capture, while driving up insurance rates and defying the world's most powerful militaries.
