"Offensive scribbling" in English on a Chinese citizen's passport by Vietnamese border officials was shameless and cowardly, China's consulate general in Ho Chi Minh City said on Wednesday.

It added that the incident had "stained the dignity of both China and its nationals".

On Saturday, a woman surnamed Zhong from Guangzhou, Guangdong province, found that offensive words had been scribbled in her passport after immigration officials handed it back to her at Tan Son Nhat International Airport in Ho Chi Minh City.

The words had been scribbled on two pages that contained the dotted line denoting China's claims in the South China Sea.

Zhong, who was on a personal visit to Vietnam, did not respond immediately at the airport because she cannot speak Vietnamese, but she later told China National Radio that she felt insulted and was "very disappointed at the personal qualities of Vietnamese officials".

The Chinese consulate general said in a statement on its website on Wednesday that it had expressed indignation, contempt and condemnation to the Foreign Affairs Office in Ho Chi Minh City.

It said China had asked Vietnam to investigate the incident and seriously punish those responsible for it, adding that Vietnam should ensure that such a case was not repeated.

According to the consulate, the Vietnamese said they would conduct an investigation. The Vietnamese embassy and consulates in China were unavailable for comment.

Earlier this month, Chinese media reports said Vietnamese border control officers refused to stamp Chinese passports with the dotted line design, with the holders being issued with separate visas on arrival.

The reports said Vietnam had adopted such measures since 2012, when China first issued new passports with the South China Sea dotted line printed in them.

China and Vietnam have overlapping claims to parts of the South China Sea.

Li Guoqiang, deputy head of the Chinese Academy of Social Sciences' Institute of Chinese Borderland Studies, said the border officials' "provocative act" reflected "long-standing irrational emotions" among some Vietnamese people regarding the South China Sea issue.

"However, this will not help to properly settle disputes between Beijing and Hanoi, and even hurts relations between the peoples of both countries," he said.

The Vietnamese government should control and manage such "misbehavior" and attempt to solve differences through government-to-government conversations, Li added.

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