Vietnam demanded on Monday that China stop sending cruise ships to the East Sea, known internationally as South China Sea, in a response to one of Beijing's latest steps to bolster its claims in the strategic waterway.
A Chinese cruise ship with more than 300 passengers visited the disputed Paracel Islands earlier this month.
"Vietnam strongly opposes this and demands that China respect Vietnam's sovereignty over the Paracel Islands and international law and immediately stop and not repeat those activities," foreign ministry spokesperson Le Hai Binh told Reuters.
"Those actions have seriously violated Vietnam's sovereignty over the Paracel Islands and international law."
Countries competing to cement their rival claims have encouraged a growing civilian presence on disputed islands in the South China Sea. The first cruises from China to the Paracel islands were launched by Hainan Strait Shipping Co in 2013.
China invaded the Paracel Islands in 1974, as American forces withdrew from the region, abandoning a naval unit of the then U.S.-backed Republic of Vietnam to a brief but bloody assault.
Vietnam's behemoth northern neighbor has illegally occupied the islands ever since; re-unified Vietnam has never relinquished its sovereignty over the Paracel Islands.
China claims 90 percent of the potentially energy-rich South China Sea. Brunei, Malaysia, the Philippines, Vietnam and Taiwan lay claim to parts of the sea, through which passes about $5 trillion of trade a year.