A top U.S. official says "uncertainties" about how China will use its increasing power may inhibit its relations with other countries, including the United States.

Deputy Secretary of State Robert Zoellick addressed the National Committee on U.S. - China Relations Wednesday in New York.

Mr. Zoellick praised the "constructive role" China played in hosting the recent six-party talks on North Korea's nuclear program. But he also encouraged China to be a responsible stakeholder in global affairs.

He said Beijing's "lack of transparency" raises questions about the purpose of China's rapid military modernization

On trade, Mr. Zoellick said China should not take for granted its access to the U.S. market.

On democracy issues, Mr. Zoellick said China's closed political system is "simply not sustainable." He said as economic growth continues, the Chinese people will want a greater say in their future.