The U.S. government believes China intervened with North Korea to tamp down growing tensions on the peninsula—a sign, officials said, that Beijing may be willing to take a tougher stance toward its erratic neighbor.

China has made a public show over the past two weeks of urging all sides to show restraint, but to Washington's chagrin has refused to blame North Korea for the sinking of a South Korean ship in March or the Nov. 23 artillery attack that killed four people on a South Korean island in the Yellow Sea.

In...