WASHINGTON, Aug 3 (Reuters) - Vice President Kamala Harris will focus on defending international rules in the South China Sea, strengthening US leadership in the region and expanding cooperation on matters of security during her trip to Vietnam and Singapore, a senior White House official told Reuters.

The trip will be Harris's first to the region, making her the only US vice president to visit Vietnam and is aimed at rallying international support to counter China's growing global influence.

The countries are critical partners for the United States because of their location, size of their economies, their trade relationship with the United States and security partnership on issues such as the South China Sea, the official said.

Former US foe Vietnam has emerged as a key partner of the United States and a vocal opponent of China's territorial claims in the South China Sea. Several countries in the region largely welcome a US presence in the face of Beijing's militarisation of the waterway and its vast coastguard and fishing fleet.

"We do not want to see any country dominate that region or take advantage of the power situation to compromise the sovereignty of others," the White House official said.

"The Vice President is going to underscore that there should be free passage for trade, throughout the South China Sea, and no single country should disrespect the right of others," the official added.

The US Navy has maintained a steady pattern of freedom of navigation operations in the South China Sea and near Taiwan but these appear to have done little to discourage Beijing.

Harris' trip will follow one that Defense Secretary Lloyd Austin made last week to Hanoi, where he sought to nudge forward security ties that have been steadily deepening amid shared concern about China's activities in the South China Sea.

It will also follow high-level talks in the northern Chinese city of Tianjin between US Deputy Secretary of State Wendy Sherman and senior Chinese diplomats, which ended with both sides signaling that the other must make concessions for ties to improve.

This will not be Harris's first meeting with leaders in Asia. She has previously held in-person and virtual bilateral meetings from the White House with the South Korean President Moon Jae-In and the Prime Minister of Japan Suga Yoshihide.

The pandemic, vaccinations and quality of vaccines will also be a top priority for the Vice President, the official said.

Last month, the United States shipped 3 million doses of the Moderna COVID-19 vaccine to Vietnam, bringing total donations to the Southeast Asian country to 5 million.

Harris will meet government, private sector and civil society leaders in both countries. She departs the United States on Aug 20, arriving in Singapore on Aug 22. She will visit Vietnam on Aug 24 and depart on Aug 26.