Vietnam has accused China of trespassing and demanded a Chinese survey ship in the South China Sea withdraw in the latest flashpoint over the disputed territory.
The two countries have long been locked in a maritime disagreement over the resource-rich waters, with the Chinese claiming nearly all of it.
But the international community has largely sided with the other claimants -- Vietnam, Philippines, Indonesia, Malaysia, Taiwan, and Brunei -- citing the importance of freedom of navigation.
On Friday, Vietnam's Ministry of Foreign Affairs criticised China's survey vessel the Haiyang Dizhi 8, and its escort ships, for entering its waters in "recent days," although it did not give exact dates.
"This is completely in the sea area of Vietnam," a statement said, adding they had contacted the Chinese multiple times, on different diplomatic channels, "to protest" the act.
The Vietnamese have "persistently requested them to... withdraw all these ships from Vietnam's waters and respect its sovereignty."