China and Vietnam, which are involved in a tense standoff in the South China Sea, are pleading their case to UN Secretary General Ban Ki-moon.
Beijing on Monday sent a paper to the UN chief, accusing Vietnam of "illegally and forcefully" disrupting Chinese oil drilling in waters claimed by both nations.
In the letter, China's Deputy UN Ambassador Wang Min said Vietnam was violating China's sovereignty and posing a "grave threat" to Chinese workers.
Vietnam later said it sent its own letter to Ban, demanding Beijing immediately withdraw the oil rig and all other ships, which it said violate Hanoi's sovereignty.
Hanoi also called on China to "create conditions" for talks on measures "to stabilize the situation and control the maritime issues between the two countries."
Both letters made reference to international treaties, including the 1982 United Nations Convention on the Law of the Sea, or UNCLOS. Both also asked Ban to distribute the documents to UN members.
China deployed a state-operated oil rig last month off the Paracel Islands in the South China Sea, within what Vietnam considers its exclusive economic zone.
The move is seen as one of China's boldest yet to advance its wide-reaching maritime claims in the South China Sea, where it also has disputes with the Philippines, Malaysia, Brunei and Taiwan.
The oil rig dispute has led to a serious deterioration of ties between communist neighbors China and Vietnam, and some fear that an accidental clash could send the situation spiraling out of control.
Hanoi has accused China of firing water cannons at and ramming Vietnamese fishing boats, including one that sank last month. Beijing said Vietnam is the aggressor and that its ships are ramming Chinese vessels.
The dispute also led to mass anti-China riots last month in Vietnam, where angry mobs destroyed Chinese-owned factories, killing at least four people, wounding dozens, and forcing the evacuation of thousands of Chinese workers.