Vietnamese immigration officers said Tuesday they were refusing to stamp entry visas into controversial new Chinese passports which feature a map of Beijing's claim to almost all of the South China Sea.
Vietnam has said the computer-chipped passports violate its sovereignty and has demanded Beijing withdraw the documents, which show the contested Paracel and Spratly Islands as Chinese territory.
"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," said the official, who did not want to be named.
A border guard in northern Lang Son province 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.
Chinese Foreign Ministry spokesman Hong Lei said Tuesday that he was not aware of Vietnam's refusal to stamp visas in China's new passports.
Beijing has attempted to downplay the diplomatic fallout from the recently introduced passports, with the foreign ministry arguing the maps were "not made to target any specific country".
Microblog users in China complained the immigration rules for the new passports were causing inconvenience and delays on arrival.
"Immigration is requesting a separate visa form. This is causing lots of trouble, and is very time consuming," one user wrote on Weibo, China's version of Twitter.
Beijing has long infuriated southern neighbours such as Vietnam with its claim to vast swathes of the South China Sea, with Chinese maps showing a "nine-dash line" that runs almost to the Philippine and Malaysian coasts.
Both the Philippines and India have also protested against the map in Beijing's new biometric passports.
India has started stamping its own map onto visas issued to Chinese visitors as the map shows the disputed border areas of Arunachal Pradesh and Aksai Chin as part of Chinese territory.
Manila, which claims part of the Spratlys, sent Beijing a formal protest letter last week, calling the maps "an excessive declaration of maritime space in violation of international law".
The South China Sea is strategically significant, home to some of the world's most important shipping lanes and believed to be rich in resources.
Other claimants to parts of the South China Sea are Brunei, Malaysia and Taiwan.