Yet the U.S. push for enforceable targets in a deal underscores the challenge of reaching a lasting truce. President Donald Trump and his deputies have criticized China for failing to live up to past promises, including a pledge to promptly open up the Asian nation's economy to more trade and investment after it joined the World Trade Organization in 2001.

Trump and Xi have given their officials until March 1 to reach an accord on “structural changes” to China's economy on issues such as the forced transfer of American technology, intellectual-property rights, and non-tariff trade barriers. It's a tight window in which to nail down deep changes to China's economic model, some of which past U.S. administrations advocated for years and U.S. lawmakers on both sides of the aisle support.

A group representing American companies doing business in China welcomed the “substantive discussions” but stressed the need to work out key details. “Progress should include a mechanism for the removal of tariffs and measurable, commercially meaningful outcomes,” the U.S.-China Business Council said in a statement. The U.S. and China have slapped tariffs on a combined $360 billion in each other's imports since July.

The USTR statement didn't say whether progress had been achieved on its main concerns. China was believed to be preparing its own separate statement on the talks. People familiar with the U.S.-China discussions said positions are closer on areas including energy and agriculture but further apart on harder issues.

Chinese foreign ministry spokesman Lu Kang said a one-day extension in talks shows that both sides are serious about the negotiations. Some disagreements remain on structural issues, and they need to be addressed when more senior negotiators meet later on, according to Chinese officials involved in the discussions who asked not to be identified.

Later this month, Lighthizer is expected to meet with Vice Premier Liu He, Xi's top economic aide who is leading negotiations for China, a person familiar with the situation said last week. Liu made a brief appearance at the talks in Beijing on Monday, boosting optimism that China is serious about making progress on a deal.

The talks were the first face-to-face meeting between the two sides since their leaders met on December 1. Prior to the meeting, China made a number of concessions to U.S. demands, including temporarily cutting punitive tariffs on U.S.-made cars, promising to open up its markets for more foreign investment, and drafting a law to prevent forced technology transfers.

China also restarted purchases of American soybeans last month, providing relief for a crop hit by Chinese retaliatory tariffs. U.S. government data shows the Asian nation scooped up more than 2.6 million metric tons of beans in December and at least another 1.3 million have been sold this year, according to people familiar with the matter. China aims to buy at least 5 million tons of American soybeans as part of the truce.

Trump is increasingly eager to strike a deal with China soon in an effort to perk up financial markets that have slumped on concerns over the trade war, according to people familiar with internal White House deliberations.

The World Bank this week cut its outlook for the global economy, as growth in trade and investment slowed and rising interest rates sapped momentum, especially in emerging markets. And the S&P 500 Index has fallen about 7 percent since Trump and Xi agreed on a 90-day truce at their meeting in Argentina last month.

“Talks with China are going very well!” Trump tweeted yesterday.