As the world looked yesterday for ways to punish the North over a nuclear test that pushes Pyongyang closer to its goal of a nuclear armed missile that can reach the US mainland, the two Koreas have quickly slid into the kind of Cold War-era standoff that has defined their relationship over the past seven decades.
A top North Korean ruling party official's warning that the South's broadcasts have pushed the Korean Peninsula "towards the brink of war" is typical of Pyongyang's over-top-rhetoric. But it is also indicative of the real fury that the broadcasts, which criticise the country's revered dictatorship, cause in the North.
Seoul resumed the cross-border broadcasts Friday for the first time in the nearly five months. When South Korea briefly resumed propaganda broadcasts in August after an 11-year break, Seoul says the two Koreas exchanged artillery fire.
South Korean troops, near about 10 sites where loudspeakers started blaring propaganda, were on the highest alert, but have not detected any unusual movement along the border, said an official from Seoul's Defence Ministry.
The South's Yonhap news agency said Seoul had deployed missiles, artillery and other weapons systems near the border to deal with possible North Korean provocation. The ministry did not confirm the report.
Officials say broadcasts from the South's loudspeakers can travel about 10km during the day and 24km at night. That reaches many of the huge force of North Korean soldiers stationed near the border and also residents in border towns such as Kaesong, where the Koreas jointly operate an industrial park that has been a valuable cash source for the impoverished North.
Seoul also planned to use mobile speakers to broadcast from a small South Korean island just a few kilometres from North Korean shores.
The South's broadcasts include news and pop music, but much of the programming challenges North Korea's Government more directly.
Seoul can't stand down easily, some analysts say, and it's highly unlikely that the North will express regret for its nuclear test, which is a source of intense national pride.
US Secretary of State John Kerry urged China, the North's biggest aid provider, to end "business as usual" with North Korea.
Diplomats at a UN Security Council emergency session pledged to swiftly pursue new sanctions. For current sanctions and any new penalties to work, better co-operation and stronger implementation from China is seen as key.
South Korean and US militaries also discussed the deployment of US "strategic assets", Seoul's Defence Ministry said. Officials refused to elaborate, but the assets will likely include B-52 bombers, F-22 stealth fighters and nuclear-powered submarines.
After North Korea's third nuclear test in 2013, the US sent its most powerful warplanes to drills with South Korea in a show of force. B-2 and B-52 bombers are capable of delivering nuclear weapons.
It may take weeks or longer to confirm or refute the North's claim that it successfully tested a hydrogen bomb, which would mark a major advance for its still-limited nuclear arsenal.
Outside experts are sceptical the blast was a hydrogen bomb, but even a test of an atomic bomb would push North Korea closer to building a nuclear warhead small enough to place on a long-range missile.
The Korea Institute of Nuclear Safety said a small amount of radioactive elements was found in air samples collected from the peninsula's eastern seas after the blast, but was too small to determine whether the North had really detonated a nuclear device.
