WASHINGTON — The Obama administration, detecting what it sees as a shift in decades of Chinese support for North Korea, is pressuring China’s new president, Xi Jinping, to crack down on the regime in Pyongyang or face a heightened American military presence in its region.

In a flurry of exchanges that included a recent phone call from President Obama to Mr. Xi, administration officials said, they have briefed the Chinese in detail about American plans to upgrade missile defenses and other steps to deter the increasingly belligerent threats made by North Korea’s young leader, Kim Jong-un.

China, which has been deeply suspicious of the American desire to reassert itself in Asia, has not protested publicly or privately as the United States has deployed ships and warplanes to the Korean Peninsula. That silence, American officials say, attests to both Beijing’s mounting frustration with the North and the recognition that its reflexive support for Pyongyang could strain its ties with Washington.

“The timing of this is important,” Tom Donilon, Mr. Obama’s national security adviser, said in an interview. “It will be an important early exercise between the United States and China, early in the term of Xi Jinping and early in the second term of President Obama.”