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.

Analysts have said North Korea is eager to re-establish contacts with Washington and Seoul in hopes of undermining the United Nations’ sanctions over its nuclear program.

The North said it would allow reunions of Korean families separated by the 1950-53 Korean War, with visits taking place at Mount Kumgang, or Diamond Mountain, during the three-day Harvest Moon Festival, when Koreans traditionally visit their hometowns. This year the festival begins Oct. 3.

Regular visits to Mount Kumgang on North Korea’s eastern coast will start “as soon as possible,” the official North Korean news agency reported, as well as visits to the ancient border town of Kaesong.

Programs allowing tour groups — predominantly South Koreans — to visit the North were expanded in October 2007 but were stopped last year when a South Korean tourist at Kumgang who apparently entered a restricted zone was fatally shot by a North Korean guard.

The announcement on Monday followed a meeting Sunday between the North Korean leader, Kim Jong-il, and the chairwoman of the Hyundai Group, the South Korean conglomerate, which is the biggest investor in the North.

The chairwoman, Hyun Jung-eun, had successfully negotiated the release of a Hyundai worker whom the North held for several months on charges of denouncing the government and encouraging defections.