Although Mr. Kim agreed to the reunions in principle, he has yet to agree to a specific date. He made an appeal on New Year’s Day for better relations, which the South dismissed as little more than empty words given the continuing advancement of the nuclear program. Last Wednesday, James Clapper Jr., the United States’ director of national intelligence, said the North had expanded the size of its uranium enrichment facility at its Yongbyon complex and had restarted a plutonium reactor that was shut down in 2007. Jane’s Defense Weekly reported that North Korea has been helping Syria enhance its missile program.

Meanwhile, officials in North Korea continue to demand that the South cancel military drills with the United States that are scheduled for later this month. It would be a mistake to do so. Land, sea and air exercises have been held regularly for decades, and they are needed to prepare for threats, especially from the North. This year the war games are unlikely to include B-2 and B-52 bombers, whose overflights last year reinforced tensions already set in motion by a North Korean nuclear test last February.

Talks involving the United States, North Korea, South Korea and three other countries on the nuclear program were suspended in 2009, and Washington, under what it calls a policy of “strategic patience,” has said they should not resume until the North takes concrete steps to show it is serious about dismantling its weapons and facilities. There is no sign of that; indeed, despite Mr. Kim’s conciliatory words, South Korean officials say they would not be surprised by another round of provocations.

The challenge for President Obama and South Korea’s president, Park Geun-hye, is to remain firm while not provoking North Korea. Though nothing so far has slowed the North’s nuclear program, or brought the two adversaries together, Washington and Seoul must remain open, and responsive, to possible avenues of cooperation, like family reunions, as tentative and remote as such gestures may seem.