Several nations have warships in the Gulf of Aden on anti-piracy patrols
A Greek-owned ship with a Ukrainian crew has been hijacked by Somali pirates south-west of the Seychelles, a seafarers' group says.
It came hours after a Portuguese warship thwarted an attack on a Norwegian vessel in the Gulf of Aden.
The warship, part of a Nato patrol, destroyed explosives its crew found when they captured the pirates.
Somali pirates have also told news agencies they seized a Ukrainian ship in the Indian Ocean late on Friday.
Reuters news agency quoted a pirate the vessel was carrying industrial equipment including white cars with the United Nations logo.
UN officials could not confirm the claim, says the BBC's East Africa correspondent Peter Greste, but pirates in the past have disrupted UN aid work by seizing ships loaded with emergency food supplies.
Saturday's attack on the Greek ship, the MV Ariana, took place about 250 nautical miles (460km) from the Indian Ocean islands.
The vessel, said to be carrying 35,000 tons of soya, was sailing from the Middle East to Brazil, said Andrew Mwangura of the East African Seafarers Assistance Programme.
It is owned by a Greek company and managed by Seven Seas Maritime in London.
In the earlier incident, the Portuguese warship, the Corte Real, sent a helicopter to help the oil tanker Kition after a distress call was made.
The incident happened about 100 miles (160km) from the Somali coast.
The Portuguese helicopter chased the pirates back to their "mother ship", or command vessel, and briefly detained about 19 pirates, a Nato spokesman said.
Explosives and grenade launchers were discovered on the mother ship when Portuguese special forces boarded "with no exchange of fire", Lt Cmdr Alexandre Santos Fernandes said.
"It was almost a kilogramme of high explosives. If used correctly it can open a hole in the hull of a ship and sink her," Lt Cmdr Fernandes said.
"It is the first time we have spotted high explosives on board a pirate ship, normally they just stick to AK-47s and RPGs (grenades)."
The 19 pirate suspects were released because they had not attacked Portuguese property or citizens.
In total, pirates are now holding 18 ships and some 300 crew for ransom.
Meanwhile, the crew of a Philippine tanker have arrived back in the Philippines for an emotional reunion with their families.
The 23 men from Stolt Strength said they had spent the past five months - the longest time Somali pirates have held hostages - in fear of being shot.
"Daily life was always a combination of fear and helplessness, hopelessness," said Captain Abelardo Pacheco.
"It was the most negative feeling one could experience," he said, quoted by AFP news agency.
After the men met their families at Manila airport, crew member Rodel Barreta said he was delighted to be home.
"Of course we're happy. Who wouldn't be happy when you're back with your family?"
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