NAIROBI - Somali pirates hijacked a Greek and a Ukrainian ship on Saturday and a NATO warship briefly detained 19 pirates armed with high explosives after foiling an attack on a Norwegian tanker in the Gulf of Aden.

Pirates said they were taking the Ukrainian ship, hijacked in the Indian Ocean with a cargo including United Nations' vehicles, to the Somali coastal town of Haradheere.

We have hijacked a ship carrying industrial equipment including white cars with the U.N. logo, our friends are on board it, a pirate who said his name was Hussein told Reuters by telephone from Haradheere.

Maritime and U.N. officials were not immediately available to confirm the hijack.

Gunmen also seized a Greek-owned bulk carrier, the MT Ariana, with a 24-strong Ukrainian crew, Andrew Mwangura of the East African Seafarers Assistance Programme told Reuters.

MT Ariana was seized at 10:00 p.m. EDT (0200 GMT) north of Madagascar en route to the Middle East from Brazil. The Ukrainian crew are said to be unhurt, he said.

The ship, which was flying a Maltese flag, belongs to All Oceans shipping in Greece and is managed by London-based Seven Seas Maritime.

Heavily-armed Somali pirates have stepped up their attacks on vessels in Indian Ocean shipping lanes and the Gulf of Aden, capturing dozens of vessels, kidnapping hundreds of hostages and raking in millions of dollars in ransoms.

Attacks have disrupted U.N. aid supplies, driven up insurance costs and forced some firms to consider routing cargo between Europe and Asia around South Africa instead.

HIGH EXPLOSIVES

NATO said a Portuguese warship went to the rescue of the MV Kition after the Norwegian crude oil tanker radioed for help on Friday afternoon as a skiff full of pirates brandishing assault rifles and rocket-propelled grenades approached.

We were the nearest warship, so we immediately scrambled our helicopter, said NATO Lieutenant Commander Alexandre Fernandes from the Corte-Real, which was about 20 nautical miles north of the tanker when the distress call went out.

The helicopter spotted the skiff and began tracking the pirates who fled to their mothership, a dhow carrying 19 heavily-armed pirates, which was later intercepted after a high-speed chase by a Portuguese escort frigate.

Eight marines then managed to board the vessel.

They surrendered immediately, said Fernandes, who added no injuries were reported and the pirates did not shoot at the Bahamas-flagged merchant vessel, the helicopter or the marines.

NATO's Fernandes said the pirates captured by the Corte-Real, which was last week recalled from other duties to fight piracy in the Gulf, were released after consultations with Portuguese authorities.

Each warship on NATO's anti-piracy mission Operation Allied Protector must comply with its national regulations on dealing with captured pirates.

Fernandes said the special forces had found the pirates were armed with four sticks of the chemical high-explosive P4A, four AK-47s and a rocket propelled grenade launcher with nine grenades.

It was almost a kilogram of high explosives. If used correctly it can open a hole in the hull of a ship and sink her, Fernandes said, adding that he did not think the explosives signaled an escalation in violence.

It is the first time we have spotted high explosives on board a pirate ship, normally they just stick to AK-47s and RPGs.

Many of the sea gangs are based in Puntland, which has been relatively peaceful compared with southern Somalia -- which has been mired in conflict for the last 18 years.

Madagascar South Africa Somalia