US President Barack Obama and China's Xi Jinping vowed to fight global warming and halt commercial cyber-theft on Friday, but exchanged sharp words on human rights and territorial disputes.

At an extraordinary joint news conference, Obama chided China on its treatment of dissidents and insisted hacking attacks on US firms must stop, even as he thanked Xi for his commitment on climate change.

The world's top two economic powers are also its biggest polluters, and campaigners hailed their joint commitment to reduce emissions as a key step towards a global climate pact before the end of the year.

This achievement was all the more remarkable given the tensions between the rival great powers over industrial espionage and China's aggressive moves to seize disputed territory in the South China Sea.

The red carpet and full ceremonial honors that welcomed Xi to the White House underlined the importance of the great powers' relationship, but the leaders made no effort to conceal the differences between them.

"We had a frank discussion about human rights as we have in the past," Obama said, branding China's authoritarian treatment of political dissidents and religious or regional minorities "problematic."

Provocatively, Obama directly cited the name of Beijing's number one bugbear -- the Dalai Lama, the exiled Tibetan Buddhist leader seen by China as a criminal separatist -- at the leaders' joint press conference.

"Even as we recognize Tibet as part of the People's Republic of China, we continue to encourage Chinese authorities to preserve the religious and cultural identity of the Tibetan people and to engage the Dalai Lama or his representatives," Obama said.

The two delegations issued a joint promise not to spy on each other's private enterprises for commercial gain, but here again, Obama used tough language, declaring: "I indicated it has to stop."

Xi protested that "China strongly opposes and combats the theft of commercial secrets and other kinds of hacking attacks."

- Disputed islands -

The Chinese leader also firmly pushed back on human rights criticism, warning reform would come on China's own timetable and without undermining its stability.

"We must recognize that countries have different historical processes and realities, that we need to respect people of all countries in the rights to choose their own development path independently," he said.

There was also a sharp exchange of views over China's bid to extend its sovereignty over the South China Sea by building bases on reclaimed islands in areas disputed by Washington's southeast Asian allies.

"Islands in the South China Sea, since ancient times, are China's territory," Xi declared. "We have the right to uphold our own territorial sovereignty and lawful and legitimate maritime rights and interests."

Obama said the disputes must be settled in accordance with international statutes, saying: "I encouraged a resolution between claimants in these areas. We are not a claimant. We just want to make sure that the rules of the road are upheld."

Against this background of discord, the agreement on climate change -- both countries signed a "joint vision" ahead of December's UN climate summit in Paris, and China committed to a domestic "cap and trade" carbon exchange -- was all the more remarkable.

China will also set aside $3.1 billion as a fund to help developing countries fight climate change.

"If the world's two largest economies, energy consumers and carbon emitters come together like this, then there is no reason for other countries, whether developed or developing, to not do so as well," Obama said.

Environmental campaigners hailed the announcement.

Rhea Suh, president of the Natural Resources Defense Council, said the United States and others should take inspiration from the Chinese measures.

"This is strong medicine. China is promising decisive action," she said. "It lays to rest the flawed argument that Chinese pollution is an excuse for US inaction."

- Out of kilter -

Xi is seen in Washington as one of the strongest Chinese leaders in decades.

Even as the Chinese economy has slowed -- calling into question Xi and the Communist Party's technocratic bona fides -- the president has tightened his grip on power.

But his assertiveness has fueled a host of disputes that US officials say risk throwing the delicately balanced relationship out of kilter.

"At any one time we are cooperating and competing," said one senior US administration official.

"What we strive for is to make sure that competition doesn't define the relationship and that competition is taking place in a way that is healthy and is fair."

Source: AFP