The Pentagon announced on Wednesday it has disinvited China from participating in the upcoming Rim of the Pacific Exercise, in a major sign of tension between the U.S. and Chinese militaries.

“The United States is committed to a free and open Indo-Pacific. China’s continued militarization of disputed features in the South China Sea only serve to raise tensions and destabilize the region. As an initial response to China’s continued militarization of the South China Sea we have disinvited the PLA Navy from the 2018 Rim of the Pacific (RIMPAC) Exercise. China’s behavior is inconsistent with the principles and purposes of the RIMPAC exercise,” Pentagon spokesman Marine Lt. Col. Christopher Logan said in a statement.

Logan also announced the Pentagon has “strong evidence” that China has deployed anti-ship missiles, surface-to-air missile systems, and electronic jammers to disputed land features in the South China Sea.

“We have strong evidence that China has deployed anti-ship missiles, surface-to-air missile (SAM) systems, and electronic jammers to contested features in the Spratly Islands region of the South China Sea. China’s landing of bomber aircraft at Woody Island has also raised tensions,” Logan said.

RIMPAC is hosted every two years by the U.S. Navy’s Pacific Fleet, and is the largest joint maritime exercise in the world.

The U.S. Navy first invited China to RIMPAC in 2014, in a gesture that was hailed at the time of growing military-to-military relations between the two countries. China rankled the U.S. by also sending spy ships to participate.

However, tensions have continued amid China’s continuing military reinforcement of land features in the South China Sea that are claimed by multiple nations.

“While China has maintained that the construction of the islands is to ensure safety at sea, navigation assistance, search and rescue, fisheries protection, and other non-military functions the placement of these weapon systems is only for military use,” Logan said.

The Pentagon directly called on China to remove military systems from the disputed South China Sea features, recalling Chinese president Xi Jinping’s promise not to militarize the area.

“We have called on China to remove the military systems immediately and to reverse course on the militarization of disputed South China Sea features,” he added.

“We believe these recent deployments and the continued militarization of these features is a violation of the promise that President Xi made to the United States and the World not to militarize the Spratly Islands.”