Mr. Lighthizer said he expected the deal to be signed in Washington during the first week of January and take effect 30 days later. He acknowledged that future phases of negotiations, which would entail some of the bigger structural changes the United States has been seeking from China, could still prove challenging to accomplish. During the negotiations, the United States repeatedly pressed China to make more transformative changes to its economy, only to be rebuffed and have the talks nearly collapse.

“There are still a lot of outstanding issues that you’re all aware of between the United States and China, which are very serious issues,” Mr. Lighthizer said. “Our sense is that we’re better off doing this in phases than to sit and make no progress at all.”

“I’m not Pollyanna,” he added. “This is going to be a very long term issue.”

Wang Shouwen, China’s vice commerce minister, said at a news conference in Beijing that the two sides had made “significant progress” and that the United States would remove tariffs “phase by phase,” suggesting that the countries had agreed to roll back more tariffs in the future when additional agreements are reached.

Mr. Wang said that both sides had agreed to complete legal reviews as quickly as possible and that an official signing was still being worked out.

The Chinese government did not echo American assertions that it would buy $50 billion of farm goods a year, saying only that purchases would increase by a “considerable margin” to meet China’s needs for goods like soybean and pork.

The trade war has taken a major toll on America’s farmers, who have seen Chinese sales of soybeans, pork and other products dry up, and Mr. Trump has consistently promised that Beijing will commit to buying $50 billion worth of farm goods as part of a trade pact. China has resumed buying some American products, in part because an epidemic is ravaging the country’s hogs, sending pork prices soaring.

“I think they’ll hit $50 billion,” he said on Friday. “They’ve already stepped it up.”

Evan S. Medeiros, a Georgetown University professor who was senior Asia director at the National Security Council under President Barack Obama, said the only thing of substance that had been negotiated — agricultural purchases for tariff relief — was shallow compared with Mr. Trump’s earlier, much grander promises.