Taiwan President Ma Ying-jeou's re-election is a relief for China and the US, but observers say he could face a tough second term, forced to balance demands from Beijing with fears he is selling out.

Beijing-friendly Ma's surprisingly comfortable victory over his China-sceptic challenger on Saturday was greeted with expressions of hope that positive momentum in the island's ties with the mainland can be maintained.

China's state media said Ma's win "may open new chances for the peaceful development" of relations, while the White House called on China and Taiwan to continue their "impressive efforts" to build ties.

Ma's outreach to China over the past four years has made the strategically vital Taiwan Straits area, which sits astride some of the world's major shipping lanes, more stable than at any other time in the past six decades.

Challenger Tsai Ing-wen of the populist Democratic Progressive Party had caused concern by suggesting she may not accept the longstanding formula under which Taiwan agrees, in a vague and non-committal way, to the idea of "one China".

"This is the best-case scenario for cross-strait relations," Chu Shulong, an international relations expert at Beijing's Tsinghua University, said of the victory for Ma and his Kuomintang party.

"Ma's victory will ensure that the stability and peaceful development between the two sides in the past four years can continue," he said.

During his first term, Ma oversaw the most dramatic thaw in mainland ties since China and Taiwan split in 1949 after a civil war, with a sweeping trade pact signed in 2010 considered a crowning achievement.

But outside economic initiatives, Hong Kong-born Ma has proceeded carefully, constantly reassuring the public that his top priority is Taiwan's sovereignty, a cautious approach that was vindicated in Saturday's vote.

"There's no mandate for moving faster than Ma has done thus far. The gradual approach while affirming Taiwan's autonomy is popular," said Clayton Dube, a Taiwan expert at the University of Southern California.

The big question is if China is satisfied with the current measured pace or if it would like more boldness in Taipei, moving from economic issues to more sensitive political ones, such a peace treaty to formally end the civil war.

Ma indicated Sunday he was aware that he faces a critical test as he balances warming China relations with domestic concerns over the pace of rapprochement.

"In the four years ahead, I'll not have the pressure of seeking re-election, but I'll have the pressure of establishing a historical legacy. Therefore I'll do my best to be a model for our country and for history," he said.

Complicating any contacts between China and Taiwan is Beijing's insistence that it has sovereignty over the island and will bring about reunification, even if it means war.

In its reaction to Saturday's vote, China's Taiwan Affairs Office said it is prepared "to join hands with all walks of life on the basis of continuing to oppose 'Taiwan independence'."

The next few months could be critical as China undergoes a complex power transition, according to Joseph Wu, a political analyst at the National Chengchi University and a former top China policy-maker when Taiwan was led by the anti-Beijing Democratic Progressive Party (DPP).

"There will be a wave of pressure before Chinese President Hu Jintao steps down in October as Hu seeks to establish his legacy for his accomplishment in cross-strait policies," he said.

"We haven't seen any change in Ma's personality or leadership style in the past four years and he will probably be too soft to resist China's pressure and defend Taiwan's sovereignty."

One the other hand, policy-making in Beijing is becoming more sophisticated, and Chinese officials are keenly aware of how a democracy like Taiwan's works.

That means that Ma is likely to enjoy some leeway in how fast to move, said John Ciorciari, a Taiwan expert at the University of Michigan.

"The Chinese government is doubtlessly pleased with his re-election and understands the political dynamics in Taipei," he said.

"China will press Ma for further engagement but will try to not to back him into a domestic political corner that would invite a DPP victory in the next presidential election."

According to this view, Taiwan and China will continue to boost their trade and investment ties, but be extremely cautious about any move that could be interpreted by the Taiwanese public as a step towards unification.

"China does not have any illusion as to how far Ma can go," said George Tsai, a political observer at the Chinese Culture University in Taipei.

"I think as long as he can create an irreversible trend for stable relations that's good enough for everybody."