Vietnam Thursday urged China to move an oil rig in the South China Sea and give up plans to drill in disputed waters off China's Xisha Islands, a day after the pro-US Vietnamese prime minister stepped down.

Newly elected Vietnamese Prime Minister Nguyen Xuan Phuc, 61, is sworn in at parliament house in Hanoi on Thursday. He is taking over from Nguyen Tan Dung as the country's new prime minister, rounding off a key leadership change among the Communist Party's top brass. Photo: AFP

But observers said they believe that in spite of Hanoi's tougher stance on the South China Sea issue, the bilateral ties might improve.

"Vietnam resolutely demands that China cancel its plan to drill and immediately remove the rig Haiyang Shiyou 981 out of this area," Vietnamese foreign ministry spokesperson Le Hai Binh said Thursday, Reuters reported.

China should take "no further unilateral action that further complicates the situation and make practical contributions to peace and stability," the spokesperson said.

On the same day, Nguyen Xuan Phuc was elected Vietnam's new prime minister at the 11th session of the 13th National Assembly of Vietnam, winning 446 of 490 valid votes, following the removal of the pro-US predecessor Nguyen Tan Dung from office on Wednesday, the Xinhua News Agency reported.

However, Nguyen Tan Dung will not be the last Vietnamese leader to show a tough stance against China, experts said.

"Vietnam has been taking a tough stance on the South China Sea issue and the new government is unlikely to compromise with China," Gu Xiaosong, an expert on Southeast Asian studies at the Guangxi Academy of Social Sciences, told the Global Times.

After the country's rig began operating in the Xisha islands, Nguyen Phu Trong, re-elected general secretary of the Communist Party of Vietnam, said Vietnam will deal with China's drilling rig at any cost, Zhuang Guotu, head of the Center of Southeast Asian Studies at Xiamen University, told the Global Times, noting that Vietnam's maritime law also makes it impossible for Hanoi to change its stance.

Located 17 nautical miles from China's Zhongjian Island in the Xisha Islands but 150 nautical miles away from Vietnam's coastline, the rig Haiyang Shiyou 981 has faced opposition and disturbance from the Vietnamese side since it started operating in May 2014.

Vietnam's maritime law, which took effect in 2013, includes China's Xisha Islands and Nansha Islands in Vietnam's sovereignty and jurisdiction, a move Beijing said would "complicate and escalate issues between the two countries," Xinhua reported.

"Even if Vietnam does not relax its stance on the disputed area, the new leadership will try to avoid conflicts with China," Gu said.

Zhuang said that the change in Vietnam's political balance may better China-Vietnam relations, as the influence of radical and pro-US leaders from the southern Vietnam is declining, making it possible for Hanoi to keep its distance from the US.