SEOUL, South Korea -- In response to North Korea's latest nuclear test, South Korea on Thursday announced it would resume cross-border propaganda broadcasts Pyongyang considers an act of war. Seoul also began talks with Washington that could see the arrival of nuclear-powered U.S. submarines and warplanes to the Korean Peninsula.

From Seoul to Washington, Beijing to the United Nations, world powers are looking at ways to punish Pyongyang for the test of what it called a new and powerful hydrogen bomb.

The loudspeaker broadcasts, which will start today, believed to be the birthday of young North Korean leader Kim Jong Un, are certain to infuriate authoritarian Pyongyang because they are meant to raise questions in North Korean minds about the infallibility of the ruling Kim family.

South Korea stopped earlier broadcasts after it agreed with Pyongyang in late August on a package of measures aimed at easing animosities that had the rivals threatening war.

Experts, meanwhile, are trying to uncover more details about the detonation that drew worldwide skepticism and condemnation.

It may take weeks or longer to confirm or refute the North's claim 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 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.

Statements from the White House said President Barack Obama had spoken to South Korean President Park Geun-Hye and to Prime Minister Shinzo Abe of Japan.

The statements said the countries "agreed to work together to forge a united and strong international response to North Korea's latest reckless behavior."