It has also asked Beijing to refrain from further complicating the situation.
"All activities of the parties on the Truong Sa (Spratly) and Hoang Sa (Paracel) Islands without Vietnam's permission are invalid," Foreign Ministry spokeswoman Le Thi Thu Hang said Thursday.
Hang was responding to a reporter's question about China sending KQ-200 maritime patrol aircraft of its navy to the Fiery Cross Reef on the Spratly Islands in the East Sea, internationally known as the South China Sea, at an online press meeting.
On April 23, Israeli ImageSat International (ISI) posted on its Twitter account a satellite image taken on April 10 of China's KQ-200 maritime patrol aircraft on the Fiery Cross Reef.
It said the use of this type of aircraft in training meant "practicing finding enemy submarines and increasing situational awareness in the maritime arena."
The ISI said it also captured "rare documentation" of an active aircraft shelter on the same reef.
"Such a concentration of aircraft can be indicative of widespread exercise in the area," ISI said.
Fiery Cross Reef is among the seven features of Vietnam's Spratly Islands that have been illegally occupied by China which later illegally added artificial islands, built runways, lighthouses, and civil and military constructions.
Vietnam has full legal basis and historical evidence to assert its sovereignty over the Hoang Sa (Paracel) and Truong Sa (Spratly) Islands, as well as rights over its waters in accordance with the United Nations Convention on the Law of the Sea (UNCLOS), Hang asserted.
In the context of the current international and regional situation, Vietnam has asked China to desist from further complicating the situation in the East Sea, Hang added.
Currently, Vietnam and other countries around the world are focusing on controlling the Covid-19 pandemic, preventing the possibility of another outbreak again. Countries are also trying to restore social and economic activities in order to mitigate the negative impacts of the pandemic, she noted.
"Activities of other countries need to comply with relevant provisions of international law, respect the sovereignty and legal rights of coastal countries, and contribute to peace, security and stability in the East Sea," Hang said.
China has been ramping up its aggressive behavior in the East Sea of late, including announcing the establishment of what it calls the "Xisha District" and "Nansha District" in so-called "Sansha City" on April 18 to manage Vietnam's Hoang Sa (Paracel) and Truong Sa (Spratly) Islands, the Macclesfield Bank and surrounding waters.
On May 8, the Foreign Ministry had opposed China’s unilateral East Sea "fishing ban", reiterating its request that Beijing not complicate the situation further.
China had seized the Paracel Islands from South Vietnam by force in 1974, and has been illegally occupying it since.