HANOI • China has urged Vietnam to investigate claims that a Chinese visitor's passport was defaced with profanities by officials at Ho Chi Minh City airport, in an apparent reference to a bitter territorial spat between the nations.

Photographs circulated on online Chinese media this week purported to show a Chinese passport with expletives scribbled on two pages next to maps of China and its hotly contested "nine- dash line", reported Agence France-Presse.

Beijing says the line demarcates its territory in the South China Sea. But Hanoi, which along with several other nations has competing claims to the strategically key waters, rejects it.

Earlier this month, an arbitral tribunal at The Hague said China's line had no legal basis, reviving the acrimony between the two countries.

A Chinese citizen, identified only by the surname Zhong, entered Vietnam on July 23 and found the passport was "defaced while crossing the border", according to a statement posted on the website of Beijing's consulate in Ho Chi Minh City.

Decrying the "shameless and cowardly act", the Consulate General raised the matter with the Vietnamese authorities, the statement said, adding "China is outraged" by the incident.

China has demanded Vietnam investigate the alleged act and "seriously punish those responsible", it said, adding "Vietnam has agreed to probe into it."

Some Vietnamese Facebook users in the country cast doubt on the authenticity of the allegation. Others reacted gleefully to an insult to a superpower which is both Vietnam's biggest foreign investor and main regional rival.

One Facebook user wrote "big applause to the immigration officers at Tan Son Nhat" airport, urging border authorities to get their pens out for all Chinese passports.

The reaction is a sign of the enmity between the two countries over the South China Sea. Hanoi routinely accuses Chinese vessels of sinking Vietnamese fishing boats, while Beijing has built landing strips and other facilities on contested shoals.

Earlier this month, Vietnam's state media reported that border authorities in the central city of Danang and southern Phu Quoc island had refused to issue visa stamps on Chinese passports with the nine-dash line.

Meanwhile, online Vietnamese newspaper VnExpress reported yesterday that check-in screens and sound systems at two major airports - Noi Bai in Hanoi and Tan Son Nhat in Ho Chi Minh City - were hacked.

Distorted information about the South China Sea as well as insults against Vietnam and the Philippines were displayed and broadcast, the report said, so the airport authorities switched them off.

Officials said the airports' security and air traffic control systems were still operating normally.

The website of Vietnam Airlines, the national carrier, was also briefly hacked.

Four Asean members - Vietnam, the Philippines, Malaysia and Brunei - have competing claims with Beijing over parts of the South China Sea. Taiwan also has claims.

Vietnam has warned that Asean risks losing relevance if it fails to step up to the security challenge in the disputed sea. The Asean Foreign Ministers' joint communique after a summit this week reiterated serious concern about the escalating maritime dispute but made no mention of the arbitral tribunal ruling.