HONG KONG  North Korea said Monday that it would open its highly militarized border with South Korea to allow periodic family reunions and group visits by tourists from the South.

The conciliatory move, coming just after the high-profile releases of two American journalists and a South Korean worker detained by the North, seemed likely to ease the growing anxiety on the Korean peninsula.

Tensions had escalated since spring, beginning with the imprisonment of the Americans, the North’s second nuclear test in May, a series of missile tests and North Korea’s refusal to re-engage in six-nation talks over its nuclear weapons.

But the North, in the announcement Monday by its official news agency, also warned the United States and South Korea about their joint military exercises, which the North said were “obviously maneuvers for a war of aggression.” It said an “annihilating” retaliation could be one consequence. Still, that kind of bellicose language is almost standard from the North and was eclipsed by its outreach about the border and tourism.