China to announce climate pledge as Xi visits Washington

China's President Xi Jinping is set to unveil a landmark pledge on climate change and curbing harmful emissions, White House officials have said.

Xi and US President Barack Obama held informal talks at the White House on Thursday evening, where they also discussed Iran's nuclear programme.

Xi, on his first state visit to the US, will be hosted at the White House for a banquet on Friday.

The visit has been overshadowed by ongoing cyber security disagreements.

Xi is in Washington DC for the latest leg of his week-long US tour.

A White House official told reporters that Xi would announce on Friday plans to launch a national carbon emissions trading scheme in 2017.

The "cap-and-trade" scheme would see Chinese companies charged to emit pollutants beyond a certain level.

China would also make a "very substantial financial commitment" to developing nations to help them move to low-polluting technologies, the Associated Press news agency quoted the official as saying.

The expected Chinese announcement follows on from a joint US-China climate pledge in late 2014.

Then, Obama set a new goal of reducing US levels by between 26-28 percent by 2025, compared with 2005 levels. China did not set a specific target, but said emissions would peak by 2030.

The two countries together produce about 45 percent of the world's carbon dioxide.

The announcement comes ahead of a major UN climate change summit in Paris later this year, and is one of the few breakthroughs expected during Xi's visit.

Little progress is expected on contentious issues including cyber security and China's territorial ambitions in South East Asia.

Xi is arriving in Washington from Seattle, where he met leaders of the business and technology world, promising to strengthen protections on intellectual property and clear obstacles to investment in China.

The US has recently alleged that the Chinese state was behind a massive data security breach of government databases as well as attacks on private firms, and Obama has said the issue risks putting "significant strains" on the important relationship.

Xi said in Seattle China was "a strong defender of cyber security" as well as a "victim of hacking", though promised to work with the US on the issue.

White House spokesman Josh Earnest told reporters on Thursday: "We put more stock in their actions than their words."