The White House on Tuesday accused China of "bullying tactics" in the increasingly tense waters of the South China Sea and said it would resist Beijing on the dispute.
"China’s recent escalation of efforts to intimidate others out of developing resources in the South China Sea is disturbing," President Donald Trump's national security advisor John Bolton tweeted.
"The United States stands firmly with those who oppose coercive behavior and bullying tactics which threaten regional peace and security."
China has been accused of deploying warships, arming island outposts and ramming fishing vessels in the resource-rich sea, also contested by Brunei, Malaysia, the Philippines and Vietnam.
Washington has repeatedly criticized China's attempts to exert growing dominance in the disputed waters, but Bolton's latest broadside comes as the two economic superpowers face off in a damaging trade war.