Computer hackers from Vietnam and China have attacked websites including portals run by each other's governments, amid a sea-border row.
The hackers replaced content on the sites with abuse and national symbols.
Vietnam's Prime Minister Nguyen Tan Dung said his country's claims to territory in the South China Sea were incontestable.
Chinese officials later warned other Asian nations to halt exploration for minerals in the area.
Both sides claim ownership of islands in the South China Sea.
The Philippines, Malaysia, Brunei and Taiwan also have rival claims in the area.
On the weekend, hundreds of Vietnamese protested against Chinese naval operations in disputed waters.
Relations between Vietnam and China have long been awkward, but public protests are extremely rare.
The demonstrations in Hanoi and Ho Chi Minh City followed a confrontation between a Vietnamese ship and Chinese patrol boats last month.
Hanoi accused a Chinese patrol of cutting the cables of a Vietnamese ship conducting seismic research about 120km (80 miles) off Vietnam's coast.
At the time of the incident, China said that Vietnamese vessels had been operating "illegally", adding that the country should "refrain from creating trouble".
On Thursday, Prime Minister Dung made his first comments on the row, saying Vietnam's sovereignty was incontestable in areas of the Paracel and Spratly island groups.
"We continue to affirm strongly and to manifest the strongest determination of all the party, of all the people and of all the army in protecting Vietnamese sovereignty in maritime zones and islands of the country," Mr Dung said in comments reported by the Thanh Nien newspaper.
Later, Vietnamese officials accused a Chinese fishing boat of once again intentionally ramming cables from an oil exploration vessel inside its exclusive economic zone.
And separately, China's ambassador to the Philippines, Liu Jianchao, warned other countries to stop prospecting for oil in the area.
The disputed islands are largely uninhabited, but the area includes important shipping routes and may contain oil and gas deposits.