The US on Wednesday demanded the release of jailed Chinese Nobel laureate Liu Xiaobo and asked China to improve on the human rights situation in the country in remarks that may put fresh strains in ties between the two world powers.However, the White House refrained from making any comment to the letter written by some of the top retired Communist leaders in China urging Beijing to abolish censorship.Liu Xiaobo is serving an 11-year jail term for inciting subversion of state power in China.