Reuters reports that Japan’s military is considering joining the United States in maritime air patrols in the South China Sea in response to China’s increasingly assertive pursuit of territorial claims, a Japanese and a U.S. source familiar with the discussions said.

While no concrete plans had yet been formulated, Japan could join U.S. patrols in the South China Sea, or operate patrols in rotation from the Japanese island of Okinawa on the edge of the East China Sea, the Japanese source said, speaking on condition of anonymity.

Japanese air patrols in an area where China is pushing disputed territorial claims, including though a massive program of land reclamation, would risk antagonizing Beijing.

But defense officials in Tokyo worry that doing nothing would allow China to eventually impose its authority over a region through which $5 trillion of sea-borne trade passes ever year – much of it heading to and from Japan.

“We have to show China that it doesn’t own the sea,” said the Japanese source.

China claims about 90 percent of the 3.5 million sq km (1.35 million sq mile) South China Sea. The Philippines, Taiwan, Malaysia, Brunei and Vietnam also claim large parts.

At a regional summit in Malaysia on Monday the Association of Southeast Asian Nations (ASEAN) released their most critical statement yet of China’s building of man-made islands on disputed reefs, which Washington and Tokyo worry will become military bases to police the area.

ASEAN said that China’s reclamation program “eroded trust and confidence and may undermine peace, security and stability”.