China has pledged to end market-distorting subsidies for its domestic industries but offered no details on how it would achieve that goal, according to three people familiar with the U.S.-China trade talks in Beijing this week.

China promised to bring all subsidy programs into compliance with World Trade Organization rules, the sources said, but U.S. negotiators reacted skeptically, in part because China has long refused to disclose its subsidies.

Without knowing exactly how the Chinese government finances industries and state-run companies, any reform promises would be difficult to enforce, said the sources, speaking on condition of anonymity because the negotiations are confidential.