US State Department

Washington alleges China's "aggressive" behaviour is threatening the region's stability and the sovereignty of some nations.

Sandra Oudkirk, the US Deputy Assistant Secretary of State for East Asian and Pacific Affairs, said there had been a range of increasingly problematic behaviour from China in the region.

She said this included predatory economic activities and investments that undermined good governance.

"While local governments continue counterterrorism operations, the Department nevertheless remains concerned about the potential for future terrorist attacks," the alert said. "U.S. citizens should always be alert to the possibility that terrorist sympathizers or self-radicalized extremists may conduct attacks with little or no warning."