BEIJING—China’s leadership pledged to ensure economic stability as its trade fight with the U.S. started to pinch growth, signaling that a bigger stimulus could be on the horizon.

A Tuesday meeting of the Politburo, whose members are China’s top arbiters of power, highlighted the external challenges faced by the world’s second-largest economy. Without specifically mentioning the trade conflict with the U.S., a statement issued after the meeting by state media made clear the brawl is a big threat to growth and stability, trumping issues such as debt control.

The Chinese economy “faces some new problems and new challenges,” the statement said. “There are obvious changes in the external environment.”

The meeting came as official data showed early Tuesday that China’s business activities faltered in July—the first official data to reflect the impact of U.S. tariffs—adding to signs that trade tensions have started to dent economic growth.

China’s economic growth has been on a controlled descent for most of this decade, propped up at times by shots of easy credit that have helped make debt a long-term threat for the Chinese economy. With growth still buoyantly above the government’s 6.5% target, Mr. Xi has taken aim at debt and other financial risks the past two years to put the economy on sounder footing.