The United States is toughening its stance over the South China Sea on China, which considers most of the energy rich sea as its own sovereign territory and has built artificial islands and converted them as military outposts.

Yesterday, U.S. Secretary of State Rex Tillerson and defense secretary Jim Mattis held a joint meeting with their Japanese counterparts in Washington and speaking after the meeting, Secretary Tillerson said, “We also discussed our concern about the security environment in the East China Sea and the South China Sea. The United States and Japan oppose any unilateral action that seeks to undermine Japan’s administration of the Senkaku Islands, and we reaffirmed that Article V of the U.S.-Japan Security Treaty covers these islands. We also oppose militarization activity in the South China Sea. Maritime disputes should be settled peacefully and maintain the freedom of navigation in accordance with the United Nations Conventions on the Law of the Sea.”

Speaking after Secretary Tillerson, the Japanese foreign minister Taro Kono said, “We shared our concerns regarding the situation surrounding East and the South China Sea. We reaffirmed that the Senkaku Islands are within the scope of Article V of the Japan-U.S. Security Treaty and that we would oppose any unilateral behavior attempting to undermine Japan’s administration of the Senkaku Islands. For the peace and stability of East Asia and the East China Sea, we will continue to cooperate with the United States. On the South China Sea, we have confirmed that we need to continue to engage ourselves and watch for any acts which would impede freedom of navigation.”

While under the Obama administration, the United States was critical of China’s island building and claims on the South China Sea, an Article V commitments were never mentioned over the disputed islands. The islands, known as Senkaku in Japan and Diaoyu in China have been a major source of friction between China and the United States. This is the first time, US fully supported Japanese claims on those islands, especially with military backing.