A group of 12 Nobel Peace Prize winners have called on US President Barack Obama to make a public call for the release of their fellow laureate, Liu Xiaobo, and his wife Liu Xia during a summit meeting with Chinese President Xi Jinping.

The laureates, led by South African Archbishop Desmond Tutu, wrote to Obama, who is also a recipient of the Noble Peace Prize, on September 2 to urge him to press the issue with Xi, the US advocacy group Freedom Now said on Wednesday.

Obama and Xi, who is on a week-long trip to the US, will meet in Washington on Thursday and Friday.

"All attempts to resolve their detentions through private diplomacy have failed," the letter released by Freedom Now said.

"We believe that unless leaders like you [Obama] take urgent action, both publicly and privately, that China will continue to believe it can act with impunity and without consequence for its behavior."

The letter said Liu Xia's health had declined "precipitously" in the past year and she should be allowed to travel abroad for medical care as she had requested.

Freedom Now founder Jared Genser, who acts as the pro-bono counsel for Liu Xiaobo and his wife, said in a note accompanying the letter that no one in the White House had ever called publicly for Liu Xia's release from house arrest, or even mentioned her name.

"This is particularly worrying because the Chinese government continues both publicly and privately to insist she isn't under house arrest at all," he said.

Veteran dissident

Liu Xiaobo, 59, a veteran dissident involved in the 1989 Tiananmen Square pro-democracy protests crushed by the army, was jailed for 11 years in 2009 on subversion charges for organising a petition urging an end to one-party rule. He won the Nobel Prize in 2010.

Last month, 10 US senators called on Obama to use his summit with Xi to take him to task for what they called an "extraordinary assault" on human rights and to call for the release of Liu Xiaobo and other dissidents.

Washington said last month that Beijing needed to improve its rights record to ensure a successful summit, but activists have accused the Obama administration of subordinating its concerns about treatment of dissidents to economic ties with Beijing.

On Wednesday, US Secretary of State John Kerry and Deputy Secretary of State Antony Blinken met Chinese activists and relatives of detained dissidents to discuss "the troubling deterioration of the human rights situation in China," the State Department said.

Kerry and Blinken expressed the Obama administration's "growing concern" about human rights in China and pledged to continue pressing for the release of political prisoners and legal reforms, including during Xi's visit.

The wife of one leading dissident posted a letter from her husband on Twitter to explain that he had asked her not to attend the meeting at the State Department.

The dissident, Gao Zhisheng, said in the letter to his wife Geng He that they should not take part in such meetings at a time when the "Communist thieves and US politicians are rubbing shoulders and fawning over each other."

Trump criticism

Xi, on a visit to Washington State, vowed on Wednesday to cut restrictions on foreign investment in China, while his chief internet regulator appeared to lay the groundwork for a basic agreement later this week on cyber warfare.

The Chinese leader started the day by publicly assuring US business leaders that he is making it easier to invest in China.

"We are working to create a new open economic system, push forward reform of foreign investment management and greatly reduce the restrictions on foreign investment," Xi told the gathering of executives in Seattle, including Apple Chief Executive Tim Cook and Berkshire Hathaway's Warren Buffett.

"GM and Ford can increase their investment in China," Xi said.

A few hours after, Boeing announced plans for an aircraft finishing centre in China, its first outside the US.

The world's biggest plane maker's long-expected move into low-cost China followed its news of a big order for some 300 planes from China, valued at about $38 billion at list prices.

But the finishing center plan has not been popular with labour unions and also attracted the ire of leading Republican presidential hopeful Donald Trump, who said it would take jobs away from the US.

Source: Agencies