The missile test, in April, was a failure, but that did little to alleviate concerns within the Obama administration that Mr. Kim was intent on pushing ahead with its nuclear weapons program. “The North is on track to build a warhead that could in a few years hit any regional target and eventually the United States,” said Evans J. R. Revere, a former United States principal deputy assistant secretary of state for East Asian and Pacific affairs.

Since the failed missile test, Mr. Kim has formalized North Korea as a “nuclear armed state” in the Constitution, another signal that the government has no intention of giving up its nuclear program, Mr. Revere said. With virtually no contact between the United States and North Korea, Mr. Revere argued, it is time for Washington to toughen its approach.

In a series of quick maneuvers, Mr. Kim, whose exact age is not known (he is believed to be 28 or 29), assumed the mantle of power immediately after his father’s death and cast aside early assumptions that his tenure would be a regency largely run by his elderly relatives.

The China News Service, a state-run agency, headlined an article last week: “Smooth transfer of power six months after Kim Jong-il’s death. North Korea enters era of Kim Jong-un.” The top North Korean Army generals, some of them in their 80s, have joined ranks around Mr. Kim, presenting a unified command, said Daniel A. Pinkston of the International Crisis Group in Seoul, who has written a forthcoming report by the group on North Korea.

At a congress of the ruling Communist Party in April, members of the Kim family were appointed to senior positions in the Politburo. The new appointees included Kim Kyong-hui, a younger sister of Mr. Kim’s father. Her husband, Chang Song-taek, also won a spot on the Politburo.

“There are no indications of any opposition to the transfer of power in the party, state or military,” Mr. Pinkston said. “Although many North Koreans are dissatisfied with the government, the barriers to collective action make it very risky and nearly impossible to organize any resistance.”