A Liberian-registered tanker laden with thousands of tons of toxic chemicals exploded in flames Tuesday, spewing out poison gases that prevented rescuers from searching for 23 missing crew members, officials said.

The 23,038-ton Maasgusar was engulfed in flames about 61 miles off the Pacific coast of central Japan."Rescue and fire teams can't even approach because of the blaze and poisonous gas from burned chemicals that the vessel was carrying," said Yoshi Nakamura, a spokesman for the Maritime Safety Agency. "We just have to stand by and wait for the fire to burn out by itself."

Nakamura said the tanker's 25,000 tons of petroleum-based chemicals - including ethylene chloride, isopropyl benzene and paraxylene - had begun to leak into the ocean.

He said the vessel was afloat but tilting to port. Most structures on the deck had burned down, he said.

The crew of 23 men, including 19 from the Philippines and four from Britain, were missing. The coast guard earlier identified the captain as Rickard James, 50, from Britain.

The rescue team found four lifeboats, but they were empty, the Maritime Safety Agency said.

The Maasgusar was heading for Yokohama from Houston when it caught fire, officials said.