By David Lawder and Jeff Mason

WASHINGTON (Reuters) - The United States expects immediate action by China on trade issues after a deal reached by the countries' leaders, including lower tariffs on automobiles and measures against intellectual property theft and forced technology transfers, a White House official said on Monday.

U.S. President Donald Trump and Chinese President Xi Jinping agreed to hold off on new tariffs for 90 days during talks in Argentina on Saturday, declaring a truce following months of escalating tensions on trade and other issues.

A White House official said the 90-day period started on Dec. 1. Earlier, White House economic adviser Larry Kudlow told reporters it would start on Jan. 1.

The Chinese offered more than $1.2 trillion in additional commitments on trade, Treasury Secretary Steve Mnuchin said on Monday. Kudlow said that figure was a broad benchmark and referred to private transactions for buying U.S. goods, subject to market conditions.

China also committed immediately to start lifting tariffs and non-tariff barriers, including reducing its 40 percent tariffs on autos, Kudlow said.

"We expect those tariffs to fall to zero," he told reporters.

Americans will get majority ownership in companies in China for the first time, which should help address top U.S. concerns about including intellectual property theft and forced technology transfers.

None of the commitments were agreed to in writing and specifics had yet to be hammered out.

Mnuchin said there was a shift in tone at Buenos Aires from past discussions as Xi offered a clear commitment to open China's markets to U.S. companies.

"This is the first time that we have a commitment from them that this will be a real agreement," Mnuchin told CNBC.

Kudlow, director of the National Economic Council, said he, Mnuchin and U.S. Trade Representative Robert Lighthizer held two private meetings with China's Vice Premier Liu He in Argentina and he told them that Beijing would move immediately on the new commitments.

"The history here with China promises is not very good. And we know that," Kudlow said. "However, I will say this: President Xi has never been this involved."

Kudlow said: "They cannot slow walk this, stall this, meander this. Their word: ‘immediately.’"

The truce boosted global markets on Monday with world stocks rising to their highest in about three weeks. On Wall Street, the benchmark S&P 500 gained nearly 1 percent, although the index had come off its earlier session highs in afternoon trading.