"This is a once in a generation opportunity," said Portman, who is visiting China for a China-US relations forum. "And if we allow this to go by, we miss an opportunity."

Developed countries' high agriculture tariffs and subsidies, aimed at protecting domestic farmers, are the key point of contention holding up an agreement among World Trade Organization (WTO) members to free up trade, Portman told a news briefing.

To make the December 13-18 WTO Hong Kong meeting a success, the European Union must adequately reduce agriculture tariffs and quotas, Portman said, blaming EU unwillingness to do for "blocking the talks."

Developing countries, which have a comparative advantage in agriculture, will not likely sign onto a trade liberalization agreement unless rich countries reduce their protection of agriculture sectors, said Portman.

"So we've got to get the tariffs down and the European Union is frankly not willing to provide meaningful market access by reducing tariffs and quotas," Portman said, building on previous comments.

"That means that the rest of the world is being asked to live with a formula that doesn't reduce tariffs and quotas adequately."

Without an agreement in agriculture, it will be difficult to get developing countries to agree to other issues, such as market opening, including cutting industrial tariffs and dropping barriers on trade in services.

"I'm concerned right now that unless we can break this deadlock on agriculture, it will be hard to make progress on all the other issues," Portman said.

"Unless the Hong Kong meeting is successful, we won't be able to help the developing world."