SEOUL, South Korea (AP) — South Korea is set to start broadcasting anti-Pyongyang propaganda across the border on Friday, believed to be the birthday of North Korean leader Kim Jong Un, as part of its retaliation for the North's recent nuclear test.

The broadcasts are certain to draw a furious response from North Korea, which considers them a declaration of war because it is extremely sensitive to any outside criticism of the authoritarian leadership of Kim. When South Korea briefly resumed propaganda broadcasts in August after an 11-year break, the two Koreas ended up threatening each other with attacks that brought them to the brink of war.

South Korean media reported that frontline troops, near sites where 11 propaganda loudspeakers are to start blaring anti-Pyongyang messages from Friday noon (0300 GMT) , had been put on highest alert. South Korea's Defense Ministry couldn't confirm the reports but said it will sternly repel any provocation by North Korea.

August's broadcasts, which began after Seoul blamed Pyongyang for land mine explosions that maimed two South Korean soldiers, stopped after the rivals agreed on a set of measures aimed at easing tension.

In the past, the broadcasts typically blared messages about alleged North Korean government mismanagement, human rights conditions, the superiority of South Korean-style democracy as well as world news and weather forecasts.

Tension flared again Wednesday when North Korea defiantly claimed it had detonated a hydrogen bomb, inviting worldwide condemnation and threats of more sanctions.

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 and unanticipated advance for its still-limited nuclear arsenal. Even a test of an atomic bomb, a less sophisticated and less powerful weapon, would push its scientists and engineers closer to their goal of building a nuclear warhead small enough to place on a missile that can reach the U.S. mainland.

President Barack Obama spoke to South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe and reaffirmed the "unshakeable U.S. commitment" to the security of the two Asian allies. Separate statements from the White House said Obama and the two Asian leaders also agreed to countries "agreed to work together to forge a united and strong international response to North Korea's latest reckless behavior."

South Korean and U.S. military leaders also discussed the deployment of U.S. "strategic assets" in the wake of the North's test, Seoul's Defense Ministry said Thursday.

Ministry officials refused to elaborate about what U.S. military assets were under consideration, but they likely refer to B-52 bombers, F-22 stealth fighters and nuclear-powered submarines.

When animosities sharply rose in the spring of 2013 following North Korea's third nuclear test, the U.S. took the unusual step of sending its most powerful warplanes — B-2 stealth bombers, F-22 stealth fighters and B-52 bombers — to drills with South Korea in a show of force. B-2 and B-52 bombers are capable of delivering nuclear weapons.

The U.N. Security Council held an emergency session and pledged to swiftly pursue new sanctions against North Korea, saying its test was a 'clear violation' of previous U.N. resolutions.

The North's claimed of a successful test drew extreme skepticism abroad.

An early analysis by the U.S. government was "not consistent with the claims that the regime has made of a successful hydrogen bomb test," White House spokesman Josh Earnest said.

South Korea's spy service said it thought the estimated explosive yield from the blast was much smaller than what even a failed hydrogen bomb detonation would produce.

Some believe North Korea might have detonated a boosted fission bomb, a weapon considered halfway between an atomic bomb and an H-bomb.

But even if the North exploded a boosted fission bomb, its explosive yield, estimated at six kilotons, showed the test was likely a failure, a South Korean defense official said Thursday. An explosion two to five times more powerful would have been reported if it were successful, the official said, requesting anonymity because of department rules.

The North's 2013 test produced an estimated yield of 6-7 kilotons of explosives, according to South Korean officials.

Fusion is the main principle behind the hydrogen bomb, which can be hundreds of times more powerful than atomic bombs that use fission. In a hydrogen bomb, a nuclear fission explosion sets off a fusion reaction responsible for a powerful blast and radioactivity.

The hydrogen bomb already is the global standard for the five nations with the greatest nuclear capabilities: the U.S., Russia, France, the U.K. and China. Other nations may either have it or are working on it, despite a worldwide effort to contain such proliferation.

Just how big a threat North Korea's nuclear program poses is a mystery. North Korea is thought to have a handful of rudimentary nuclear bombs and has spent decades trying to perfect a multistage, long-range missile to carry smaller versions of those bombs.

Some analysts say the North probably hasn't achieved the technology needed to make a miniaturized warhead that could fit on a long-range missile capable of hitting the U.S. mainland. But debate is growing on just how far the North has advanced.

To build its nuclear program, the North must explode new and more advanced devices so scientists can improve their designs and technology. Nuclear-tipped missiles could then be used as deterrents and diplomatic bargaining chips — especially against the U.S., which Pyongyang has long pushed to withdraw its troops from the region and to sign a peace treaty formally ending the Korean War.

U.S. aircraft designed to detect evidence of a nuclear test, such as radioactive particulate matter and blast-related noble gases, could be deployed from a U.S. base on the Japanese island of Okinawa. Japanese media said Tokyo mobilized its own reconnaissance aircraft over the Sea of Japan to try to collect atmospheric data.