SEOUL, South Korea — North Korea said it conducted a powerful hydrogen bomb test today, a defiant and surprising move that, if confirmed, would be a huge jump in Pyongyang’s quest to improve its still-limited nuclear arsenal.

South Korea’s spy agency and outside nuclear experts cast strong doubt, however, saying the estimated explosive yield from North Korea’s fourth nuclear explosion was much smaller than what even a failed H-bomb detonation would produce.

The doubts didn’t stop jubilation and pride in Pyongyang. A North Korean television anchor, reading a typically propaganda-heavy statement, said a test of a “miniaturized” hydrogen bomb had been a “perfect success” that elevated the country’s “nuclear might to the next level.” State media later crowed that its “H-bomb of justice” lets it stand firm against U.S. aggression.

A large crowd celebrated in front of Pyongyang’s main train station as the announcement was read on a big video screen, with people taking videos or photos of the screen on their mobile phones and applauding and cheering.

In Seoul and elsewhere there was high-level worry. South Korean President Park Geun-hye ordered her military to bolster its combined defense posture with U.S. forces and called the test a “grave provocation” and “an act that threatens our lives and future.” Japanese Prime Minister Shinzo Abe said, “We absolutely cannot allow this.”

Washington and nuclear experts have been skeptical about past North Korean claims about H-bombs, which are much more powerful and much more difficult to make, than atomic bombs. A confirmed test would further worsen already abysmal relations between Pyongyang and its neighbors and lead to a strong push for tougher sanctions on North Korea at the United Nations. The Security Council quickly announced an emergency meeting.

Whatever the type of the test, North Korea’s fourth nuclear explosion will likely push Pyongyang’s scientists and engineers closer to their goal of building a bomb small enough to place on a missile that can reach the U.S. mainland.

A successful H-bomb test would be a big new step for the North.