A human rights watchdog has slammed the U.N. secretary-general for failing to press China's president to release an imprisoned Nobel Peace laureate, accusing Ban Ki-Moon of caring more about his own re-election than the protection of dissidents.

Ban asked China to play a greater role in solving African crises during a meeting Monday with President Hu Jintao, but did not discuss the country's human rights record or its imprisonment of Liu Xiaobo.

Rights advocates have urged Ban to join other world leaders in publicly expressing concern over Liu's imprisonment, as well as the treatment of his wife, Liu Xia, who has been under house arrest since the award was announced last month.

Liu Xiaobo, a 54-year-old literary critic, is serving an 11-year sentence for subversion imposed in December after he co-authored a bold appeal known as Charter 08 calling for reforms to the country's single-party communist political system.

Philippe Bolopion, U.N. advocacy director for Human Rights Watch, accused Ban of failing "in one of his most basic duties, which is to be a clear voice on human rights."

The secretary-general's repeated refusal to call for Liu's release is "inconsistent" with his frequent calls for the release of Myanmar's detained pro-democracy leader Aung San Suu Kyi, another Nobel Peace Prize winner, he said.

"This looks like a misguided attempt to secure Chinese support for his re-election and overlooks the fact that the world needs a courageous and outspoken secretary-general," Bolopion told AP in New York. "The fact that his office doesn't even claim that human rights issues were raised in private is particularly shocking, especially coming from a secretary-general who often defends his timid public statements by tough talk behind the scenes."

At U.N. headquarters in New York, spokesman Martin Nesirky told reporters that human rights did not come up in Ban's meeting with Hu because there were many other issues to discuss. Nesirky added that the world body's previous statement on Liu — which did not call for his release but said Ban has long advocated for improvement on human rights — still stands.

Ban's five-year term ends on Dec. 31, 2011, and there is widespread speculation that he will seek a second term.

According to the U.N., Ban and Hu "discussed a wide range of issues, including climate change, the Millennium Development Goals, the Korean Peninsula and Africa." Nesirky said Ban specifically mentioned that China could play a greater role in the conflicts in Somalia and Sudan.

China has built strong economic ties in Africa in recent years, though many have criticized its role on the continent, suggesting Beijing is too eager to ignore abuses committed by the regimes it does business with.

A recent U.N. report alleged that Chinese ammunition was sent to the conflict-wracked Sudanese region of Darfur in violation of a U.N. arms embargo.

No news conference is planned during Ban's visit and he appears unlikely to raise Liu's case in keeping with his low-key style and China's immense influence in the U.N. as one of five veto-wielding permanent members of the Security Council.

Ban's public comments in China have been limited thus far to praise for the country's staging of the just-concluded World Expo and calls for more sustainable development.

Ban was scheduled to meet China's second most powerful official Wu Bangguo, chairman of the National People's Congress, on Tuesday.