Meanwhile, Japan says it may also send naval warship to help combat piracy off the coast of Somalia.

Prime Minister Taro Aso said his government may issue an order for "maritime policing activity" in line with the Self-Defence Force Law as early as January with a view to providing protection to Japanese ships from pirate activities, with the operation beginning sometime in February.

"Each nation is taking measures. So, Japan should also take its own steps. It is inappropriate to say that we cannot help foreign national ships in danger while we protect Japanese national ships. So, I think we need to revise the bill and put the proposal at the next parliamentary session. We will probably put it forward as maritime policing activity."

If realised, it would be the first overseas deployment by Japan of a defence force warship for policing activities at sea.

Such a deployment could be politically problematic as Japanese self defence force activity is restricted basically to safeguarding Japanese-registered ships or Japanese nationals under threat.

Aso expressed his intention to revise the legislation to allow Japanese warships to act when non-Japanese vessels are threatened.

Japan's government said no Japanese ships have been hijacked this year, but pirates have fired at three Japanese vessels. No one was injured.

Pirates have made an estimated 30 million US dollars hijacking ships for ransom this year, seizing more than 40 vessels off Somalia's three thousand kilometer coastline.