Updated

Japan's prime minister vowed at a memorial ceremony Tuesday to find the remains of the estimated 12,000 soldiers who are still missing from the battle of Iwo Jima, one of the bloodiest and most symbolic campaigns of World War II.

Naoto Kan, who has set the ongoing search for Iwo Jima's dead as a top government priority, made the comments at a ceremony to inter 822 sets of remains recovered last year — the largest annual figure in four decades.

"There remain many fallen soldiers," he said. "We vow to find them as soon as possible. We will do all we can to facilitate searches for them."

The number of remains has increased dramatically because of the discovery of two grave sites on the tiny island, now known as Ioto, which was what it was called by residents before the war.

Virtually all the Japanese soldiers tasked with defending the rugged crag were killed in the battle, which claimed 6,821 American and 21,570 Japanese lives.

Dozens of remains are recovered every year, but about 12,000 Japanese, along with 218 Americans, are still classified as missing in action and presumed killed on the island.

Iwo Jima was seen as key to the United States because it had an early warning radar station and three airfields used by Japanese fighter planes that posed a threat to U.S. bombing raids on Tokyo and Japan's main islands.

The U.S. wanted the airfields for its fighter escort planes.

Fighting began on Feb. 19, 1945, but Iwo Jima was not declared secured until March 26. Japan surrendered in August of that year, after the atomic bombings of Hiroshima and Nagasaki.

Generally ignored since the war, the island, 700 miles (1,100 kilometers) south of Tokyo, has been left largely untouched and is now uninhabited except for a few hundred troops at a small Japanese military outpost.

But Kan's government — inspired in part by the success in Japan of the 2006 Clint Eastwood movie "Letters from Iwo Jima" and concerned that time is running out — has made a strong effort to bring closure on Iwo Jima by stepping up the civilian-run mission to recover all of the Japanese dead.

That project began last July and took a big step forward in October, when two mass graves that may hold the remains of more than 2,000 Japanese soldiers were discovered by search teams.