The East China Sea has been contested for decades by South Korea, China and Japan, all seeking to tap huge oil, gas and mineral reserves believed be buried under the seabed.

With their extended continental shelves overlapping in the northeastern part of the ocean, the Asian powers are laying formal claims with a U.N. body in charge of sea shelf carve-up.

The three-way jousting recently took a twist. Seoul and Beijing are working on a joint submission to counter Tokyo's increasingly aggressive push in maritime issues.

The unlikely alliance with China could mark a turnaround for Korea, which has lacked assertiveness against its powerful neighbors.