“Re-entry is a real challenge,” said Gary Samore, a Harvard scholar who served as Mr. Obama’s top adviser on weapons of mass destruction during his first term. “There is a lot of heat, and a lot of vibration” as a warhead re-enters the atmosphere, aimed at its target. “You have to do live testing to see if it works,” he said. “It’s not something you can do through simulation.”

But for the North, the missile and nuclear technology may not be intended as much for military use as for a bargaining chip — the leaders presumably understand what would follow if they actually attacked the United States or one of its treaty allies in the Pacific. Each nuclear and missile test is meant to show that seven decades of sanctions and containment have failed. And each one amounts to an advertisement for the world’s most destructive weapons.

Among North Korea’s biggest customers for missile technology is Iran. While there is no evidence that the North has ever sold nuclear technology to the Tehran government, it supplied a nuclear reactor to Syria. The reactor was destroyed in a September 2007 attack by Israel.

A warhead, even an untested one, could become the ultimate export for a starving nation. But it would also be a huge risk for the North; President George W. Bush, soon after the North’s first nuclear test eight years ago, warned the country that it would be held responsible for any nuclear incident in which its weapons were used.

Mr. Obama had Robert M. Gates, then the defense secretary, issue a similar warning. But the administration’s strategy has been to largely ignore the North, refusing to acknowledge it as a nuclear state or re-engage in negotiations that Mr. Gates warned could amount to “buying the same horse again,” meaning making concessions for another temporary halt in the nuclear program, or resumed inspections.

“We remain open to dialogue with North Korea, but there is no value in talks just for the sake of talks,” Secretary of State John Kerry said on Friday after meeting with South Korea’s foreign minister, Yun Byung-se, at the State Department. “North Korea must demonstrate that it is serious about denuclearization,” he said.

The North, for its part, has been by turns seeking new talks and issuing statements that its nuclear capabilities are here to stay, and will be steadily improved. In the past week alone, it has opened fire along the demilitarized zone and released one of three Americans being held on thin charges. “What they speak and what they do seem to be inconsistent,” Mr. Yun said on Friday.

Mr. Kerry has suggested that the United States was looking for ways to re-engage with the country, though such efforts have always been treated with skepticism at the White House. That did not deter him on Friday. “The mere entering into talks is not an invitation to take any actions regarding troops” that the North wants removed from South Korea, “or anything else at this point,” Mr. Kerry said. “The first thing you have to do is come to a competent, real, authentic set of talks about denuclearization,” he said, “and that is the prerequisite.”