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.