Vietnam s President Vo Van Thuong and US President Joe Biden shake hands during a meeting at the Presidential Palace in Hanoi on September 11, 2023. AFP
President Joe Biden and Vietnam's Communist Party chief Nguyen Phu Trong said the competing claims on the strategic waterway must be settled under international norms.
Beijing claims almost the entire sea, through which trillions of dollars in trade passes annually, and has ignored an international court ruling that its assertion has no legal basis.
"The leaders underscored their unwavering support for the peaceful resolution of disputes in accordance with international law, without the threat or use of force," Biden and Trong said in a joint statement.
They also called for "freedom of navigation and overflight and unimpeded lawful commerce in the South China Sea".
The statement came a day after Biden and Trong struck a deal to deepen cooperation, widely seen as a way to counter China's growing assertiveness in the region.
Washington is at loggerheads with Beijing on a range of issues including trade, security, human rights and climate change and is looking to boost its network of allies to counter Chinese influence.
Vietnam, which fought a war with China between 1979 and 1988, is wary of its giant northern neighbour, and is one of a handful of countries with claims on the many islets and outcrops that dot the South China Sea.
Last week the Philippines accused Chinese Coast Guard and "militia" boats of harassing two of its own coast guard vessels as they took supplies to Filipino troops on the Second Thomas Shoal.
The Philippine Navy deliberately grounded an old ship on the shoal in 1999 to check China's advance in the waters.
China deploys hundreds of vessels to patrol the South China Sea and swarm reefs.
The Philippines, a longtime US ally, has outposts on nine reefs and islands in the Spratly Islands -- which Vietnam also claims along with the Paracel Islands.
Manila says Chinese coast guard and navy ships routinely block or shadow Philippine boats in the contested waters.
Tensions between Manila and Beijing flared last month when China Coast Guard vessels used water cannon against a Philippine resupply mission to the reef, preventing one of the boats from delivering its cargo.