Beijing slammed Hanoi on Thursday for "making irresponsible accusations" against China on the international stage, following Vietnam's decision a day earlier to consider pursuing legal action over China's islands in the South China Sea.

The issue is being internationalized and complicated, observers said, as Vietnam may align with the Philippines and follow Manila's example and file a case at an international court, a development one analyst said indicated a "front" was being formed against China.

During a visit to Manila on Wednesday, Vietnamese Prime Minister Nguyen Tan Dung said Hanoi was considering various "defense options" against China, including legal action, Reuters reported.

Dung did not elaborate on the other options being considered, but at a joint news conference with Philippine President Benigno Aquino Dung said Vietnam and the Philippines "call on countries and the international community to continue strongly condemning China and demanding China immediately end the violations". However, Aquino did not mention the territorial tensions with China in his speech.

Foreign Ministry spokesman Hong Lei said at a news conference on Thursday that Vietnam is simply confusing right and wrong.

"As for who is challenging another country's sovereignty, who is creating tensions and who is sabotaging peace and stability in the South China Sea, facts speak louder than words," Hong said.

Last year, Manila submitted a case to a UN arbitration tribunal in The Hague challenging China's claims in the South China Sea. Beijing has refused to participate, insisting that the best way to resolve matters is bilateral talks.

On Thursday, Vietnamese Deputy Prime Minister Vu Duc Dam said the country is closely watching how the Philippines fares in an international court over its maritime dispute with China, according to Reuters.

Wu Shicun, president of the National Institute for South China Sea Studies, said: "It is pretty obvious that Vietnam and the Philippines are forming a front to confront China on the South China Sea issue as the two countries have both been stirring trouble recently in regional waters and have demanded that ASEAN release a statement on this issue.

"Beijing's measures to respond to the apparently anti-China alliance forged by Vietnam and the Philippines should vary given the different conditions of the two countries," Wu said.

"Bilaterally, China and Vietnam have open and smooth communication channels, which ushers in the possibility for more diplomatic consultations to solve problem. For the Philippines, China should take multiple measures, including economic ones, to pull Manila back to the negotiating table," Wu said.

Li Guoqiang, deputy director of the Center for Chinese Borderland History and Geography at the Chinese Academy of Social Sciences, said Hanoi and Manila tend to perceive China as a common opponent and therefore share similar policy options, but it is unlikely that the two countries will frame an "anti-China coalition".

"Vietnam and the Philippines have different interests in the South China Sea. They are playing different roles and have a different influence in the region," Li said. "A coalition against China will risk great damage to their ties with China, which neither of them can afford."

"In the big picture, the two countries should realize that the South China Sea does not account for the entirety of relations," he added.

On May 2, Vietnam sent military ships to harass Chinese oil drilling operations around Zhongjian Island in the Xisha Islands, well within China's territorial waters. Anti-China protests in Vietnam followed and soon turned violent, claiming the lives of two Chinese citizens and injuring hundreds of others.

"China repeatedly asked the Vietnamese to stop the violence. However, instead, they turned a blind eye," Hong said.

Hong said Vietnam should immediately end the maritime harassment, punish the perpetrators of the riots and compensate for the property damage.

Reuters contributed to this story.