In visits to Singapore and Vietnam, Harris charged China with bullying its Southeast Asian neighbors, triggering sharp rebukes from Beijing, which accused the U.S. of meddling in regional affairs and disrupting peace.
"We welcome stiff competition, we do not seek conflict, but on issues such as the South China Sea, we are going to speak up," Harris told a news conference in Vietnam's capital, Hanoi.
"We are going to speak up when there are actions that Beijing takes that threaten the rules-based international order," she added.
Harris also said while evacuations underway in Afghanistan were dangerous and difficult, the "mission... must be seen through."
The United States and allies urged people to move away from Kabul airport on Thursday due to the threat of a terror attack by Islamic State (IS) militants as Western troops hurry to evacuate as many people as possible before an Aug. 31 deadline.