U.S. Vice President Kamala Harris will focus on defending international rules in the South China Sea, strengthening Washington's regional leadership and expanding security cooperation during her trip to Vietnam and Singapore this month, a senior White House official has said.

Harris will be the first U.S. vice president to visit Vietnam as Washington seeks to bolster international support to counter China's growing global influence.

The U.S. official said Washington saw both countries as critical partners given their locations, the size of their economies, trade ties and security partnerships on issues such as the South China Sea, which China claims almost in its entirety.