Image copyright People's Daily

China has asked Vietnam to investigate reports that a Chinese visitor's passport was handed back with obscenities written on two pages.

Pictures show a four-letter word written on the pages that contain a map including China's "nine-dash line".

The line marks China's claim to territories in the South China Sea.

Those claims, which overlap with Vietnam's, were recently rejected by an international tribunal.

The tourist, surnamed Zhong and from China's Guangdong province was entering Vietnam in Ho Chi Minh City. She told local media that she was "very disappointed at the personal qualities of Vietnamese officials".

The Chinese consulate in Ho Chi Minh City said in a statement the act was "shameless and cowardly", adding that it had "stained the dignity of both China and its nationals".

China has asked Vietnam to investigate the incident and "seriously punish" those responsible for it, the statement said. An investigation is to be conducted, according to Vietnamese officials.

Why is this an issue now?

New passports were issued by Beijing in 2012 with revised maps to include the "nine-dash line", which China uses to demarcate its territorial claims.

China claims almost all of the South China Sea, including reefs and islands that are also claimed by other countries, and has controversially been engaging in island-building and naval patrols.

It has said it does not recognise an international tribunal ruling in July that said its claims had no legal basis.

Vietnamese border officials announced earlier this month that they were refusing to stamp the new passports, saying Chinese visitors would be issued with a separate on-arrival visa instead.

"By issuing separate visas, Vietnamese authorities can avoid directly stamping the passport, thus demonstrating Vietnam's stance of not recognising the nine-dash line in any form," the vice-chairman of Quang Ning province's People's Council, Mr Nguyen Xuan Ky told news outlet Tuoi Tre.

Customs officers at Vietnam's Da Nang airport have reportedly also confiscated maps featuring the nine-dash line from Chinese passengers.

Is this the only case of anti-Chinese sentiment in Vietnam?

Image copyright Getty Images Image caption Vietnamese border officials announced earlier this month that they refused to stamp the new passports

A provincial Vietnamese television station stopped airing Shanghai Bund, a Chinese remake of a Hong Kong series after the show's lead actor voiced his support for Beijing's claims in the South China Sea.

Some 20 people were also detained in Hanoi in July while protesting against China's rejection of the tribunal decision.