Japan passed a law that allows its navy wider scope to use force and to protect foreign-flagged ships off Somalia, officials say this Friday. Critics from the opposition say that the move further erodes the country's pacifist constitution.

The new law allows Japan's Maritime Self-Defence Force to protect any commercial ships threatened by pirates, not just those sailing under the Japanese flag or carrying Japanese nationals or cargo.

It is the first international policing assignment for Japan's military since World War II. Last March, the country joined multinational forces in the fight against pirates who are primarily attacking ships in the Gulf of Aden, a key route leading to the economically strategic Suez Canal.

It also widens the navy's rules of engagement and allows it to fire at the hulls of pirate vessels that approach other ships, but not at the pirates themselves. The can resort to this action only after repeated warnings.

Until now, the Japanese parliament had to be consulted every the military wanted to launch an attack.

The opposition, led by the Democratic Party of Japan (DPJ), had called for a revision to the bill. They want to strengthen the role of the coast guard instead of the military in anti-piracy activities far from Japanese waters.

Conservative Prime Minister Taro Aso, who faces an election this year, had strongly promoted the bill, arguing Japan should play a greater role in international security.

The premier stressed that security of maritime transport is vitally important for Japan, a resource-poor island nation that draws much of its energy needs from the Middle East.

Japan has sent 400 military in total and has also dispatched two maritime surveillance aircraft and scores of military personnel to beef up its anti-piracy mission.

Japan's major overseas military missions in recent years, including its participation in Iraq and UN peacekeeping missions, have focused on logistical and support operations such as refuelling, transport and reconstruction.

Watch a movie of Canadian forces trying to free a hijacked Norwegian cargo ship.