China is asking Vietnam to investigate reports that a border agent at the Ho Chi Minh City airport defaced a Chinese passport.
Images in state media reports show the obscene words scribbled twice over maps of the contested South China Sea.
The Chinese consulate in Ho Chi Minh City said in a statement yesterday it strongly condemned the "shameless and cowardly" act and asked Vietnamese officials to investigate and punish the agent.
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China parked an oil rig near Vietnam's central coast.
China issued new passports starting 2012 with revised maps to include the "nine-dash line," demarcating Beijing's claim to nearly the entire South China Sea.