After years of simply dumping it into the sea, Japan is struggling to find new ways to get rid of almost 1,000 tons a year of unexploded ordnance and unused ammunition from World War II, with international environmental controls set to tighten.

Japan is one of 80 countries that ratified the 1972 London Convention pledging to clean up the world's oceans by stopping such dumping.

While a 1996 protocol based on the pact is widely expected to go into effect sometime next year, Japan is still disposing well over half of its obsolete arms into coastal waters, according to Environment Ministry spokesman Masafumi Takahashi.

Last year, the Defense Agency collected some 980 metric tons of unusable arms, including unexploded bombs, ammunition, artillery shells and other weapons. Many of them were left over from U.S. air raids or discovered in stashes hidden by the Japanese army during the closing days of the war.

Of this haul, 616 tons were loaded onto navy ships and disposed of at six designated offshore dumping sites, agency records show.

"Such practices will eventually have to come to a stop," Takahashi said, if Japan is to observe the 1972 agreement.

One of the challenges of doing so, however, is cost.

While dumping waste into the sea costs almost nothing, safely disposing of 1 metric ton of ordnance inland costs 1.5 million yen to 3 million yen, according to the agency.

Last year, some 60 metric tons of unexploded bombs were discovered. Only 30 metric tons were consigned for disposal by private companies, which use a variety of methods, including detonation, incineration, chemical treatment and vaporization.

The 1972 London Convention pledged to prevent marine pollution by stopping the dumping of man-made waste into the oceans. In 1996, it was amended and tightened to ban the disposal of radioactive waste and incineration at sea.

Japan has yet to ratify the 1996 protocol.