China said Thursday that it helped the United States search for an American sailor who fell into the South China Sea -- a rare cooperative gesture in the disputed waters.

"The Chinese Liuzhou naval vessel, which performs combat preparation duties in nearby waters, carried out operational coordination with the US side in the spirit of humanitarianism and in accordance with the 'Code for Unplanned Maritime Encounters'," the Chinese defence ministry said in a statement.

The sailor went missing Tuesday during a joint US-Japanese drill in the South China Sea.

Japan's Maritime Self-Defense Force, the country's navy, confirmed two of its vessels as well as helicopters have been taking part in the search.

The two nations called off their routine joint operation to focus on the search, a Japanese navy spokesman told AFP.

The sailor's name is being withheld while the search is ongoing and the exact location of the search was not disclosed.

Japanese and US naval forces have been holding drills in the contested South China Sea in the face of Beijing's increasing assertiveness over its maritime claims in the region.

Beijing has pursued claims on nearly the entire territory of the South China Sea by building military facilities and conducting naval exercises -- despite partial counter-claims from nations such as the Philippines and Vietnam.

China is also involved in a simmering territorial row with Tokyo over disputed islands in the East China Sea.