More than 200 Vietnamese websites have been attacked and some defaced with Chinese flags, an Internet security firm said Friday as a maritime dispute raises tensions between the countries. The ministries of agriculture and foreign affairs are among those targeted since the beginning of June, said Nguyen Minh Duc, director of the state-linked Bach Khoa Internetwork Security Centre (BKIS). Hackers sometimes left \"information in Chinese and Chinese flags,\" he told AFP. \"We don\'t yet know if it concerns Chinese hackers,\" Duc said. Foreign ministry spokeswoman Nguyen Phuong Nga told reporters on Thursday that \"a number\" of Vietnamese websites had been hacked, including one from the ministry\'s translation centre. Tensions between China and Vietnam are at their highest level in years after Hanoi accused three Chinese marine surveillance vessels of cutting the exploration cables of an oil survey ship in May inside its 200 nautical mile exclusive economic zone. On Thursday Vietnam alleged a similar incident, saying a Chinese fishing boat rammed the cables of another oil survey ship in its waters. Beijing countered by warning Vietnam to halt all activities that it says violate its sovereignty in disputed South China Sea waters. The two countries have long-standing disputes over the potentially oil-rich Paracel and Spratly archipelagos and surrounding waters. Hundreds of people held a peaceful anti-China protest outside Beijing\'s embassy in Hanoi on Sunday, the largest action of its kind since 2007. Protests are rare in authoritarian Vietnam but web users have been debating whether or not to hold another rally.