UNITED NATIONS—China lashed out at the U.S. on Monday over North Korea, saying Washington was ratcheting up tensions with Pyongyang and violating Security Council resolutions calling for restraint and dialogue.

China’s ambassador to the U.N., Liu Jieyi, criticized the U.S. for its plan to impose unilateral sanctions that don't have the Council’s backing and for publicly saying that Washington was keeping all options on the table. Joint U.S. and South Korean military exercises in the Korean Peninsula, he added, risked destabilizing...