Vietnamese border guards are refusing to stamp entry visas into new Chinese passports, which feature a map of the Chinese territory of the South China Sea.
Vietnam has said the computer-chipped passports violate its “sovereignty”, and has demanded Beijing withdraw the documents.
"We do not stamp the new Chinese passports," said an official at Hanoi's Noi Bai Airport, the country's main international gateway.
"We issue them a separate visa." A border guard in northern Lang Son province of Vietnam said they were also not stamping the new passports, but issuing separate visas to Chinese arrivals.
Even with the new passports, however, "Chinese citizens can still travel normally through the border gate," the guard added. Both the Philippines and India have also protested against the map in Beijing's new biometric passports. The United States plans to raise the issue with Beijing.
"We do have concerns about this map, which is causing tension and anxiety between and among the states in the South China Sea," State Department spokeswoman Victoria Nuland said.
Ms Nuland said the United States would accept the passports as a valid travel document, as it was up to countries "to decide what their passports look like as long as they meet international standards."