Ms Harris made the comment after visiting Vietnam to stress US commitment to Asia, which has been criticised given parallels with the superpower's retreats from Saigon and Kabul.
'We need to find ways to pressure and raise the pressure, frankly, on Beijing to abide by the United Nations Convention on the Law of the Sea,' said Ms Harris.
Ms Harris also accused China of intimidation in the resource-rich waterway.
China hit back, accusing the US of hypocrisy in attempting to intimate countries in the region in its scheme to contain China.
Ms Harris' arrival in Vietnam was delayed due to an anomalous health incident in Hanoi.
Sometimes referred to as Havana syndrome, it's not clear what causes the syndrome, leading to unproven allegations that Russians or others used high-intensity electronic devices to physically harm US diplomats.
Meanwhile, Ms Harris emphasised Washington's commitment to the region as she opened a branch of the US Centres for Disease Control (CDC) in Hanoi.
Ms Harris declared one million Pfizer vaccines will be donated to Vietnam from the US.