Somali pirates are believed to have made about $50m last year
Pirates have seized two European-owned tankers off the coast of Somalia in the past day, officials have said.
The Greek-owned vessel Nipayia, with 19 crew on board, was seized on Wednesday, the Nato Shipping Centre said.
The Norwegian-owned tanker Bow Asir was captured by pirates on Thursday. It had a crew of 27 on board, the Norwegian Shipowners' Association said.
Warships from more than a dozen nations currently patrol the region, following a spike in pirate attacks in 2008.
A European Union naval taskforce of seven warships reports some success in preventing other seizures, BBC diplomatic correspondent James Robbins says.
But a spokesman for the taskforce stressed that patrolling more than a million square miles of ocean is a huge undertaking, our correspondent reports.
Some ships are taking successful counter-measures and outrunning the pirates, he says, while others are sailing in groups along sea corridors where they can be offered better protection.
Nato has said five extra warships will join international protection efforts in the coming days.
The 23,000-tonne Bow Asir, which is registered in the Bahamas, is reported to be a chemical tanker. It is not known what cargo it was carrying at the time of the hijack.
Polish foreign ministry spokesman Piotr Paszkowski said the ship's Norwegian owner had informed the ministry that five sailors on board were Polish, the Associated Press news agency reports.
The vessel has a Norwegian captain, the Norwegian Shipowners' Association said. It is believed to have been seized by a group of 16 to 18 pirates.
The 9,000-tonne Nipayia was captured about 450 miles from the southern Somali port of Kismayo, a spokesman for the EU Naval Force (EU Navfor), told the BBC.
The crew in both hijacked vessels were believed to be unharmed, the spokesman said.
The Nipayia has a Filipino crew, according to the website of the EU's Maritime Security Centre.
Both tankers are far smaller than the Sirius Star, the Saudi supertanker that was captured by Somali pirates last November carrying 2m barrels of oil. It was released in January.
The Nato Shipping Centre said three other vessels had reported being pursued by small boats on Wednesday and Thursday.
Somali pirates also hijacked a yacht with two people on board after it left the Seychelles in the Indian Ocean, the Seychelles state broadcaster said on Wednesday.