South Korea has raised its military alert to the highest level at locations along the border with North Korea, following Pyongyang's announcement this week that it had successfully tested a hydrogen bomb, reported Reuters, citing South Korean news agency Yonhap. South Korea is also scheduled to resume propaganda broadcasts across the border Friday.

Seoul has said it would resume the broadcasts, which have angered North Korea in the past, because Wednesday's test — the fourth for North Korea -- violated an agreement to ease tensions between the nations. The broadcasts largely sparked an exchange of artillery fire across the border in August.  Troops in South Korea were moved into a high-alert “fully armed, wartime state” following the August incident.

The broadcasts are expected to begin Friday at noon. "The North's fourth nuclear test is a grave violation" of the August deal to ease tensions, said Cho Tae-yong, South Korean deputy chief of the presidential office of national security, according to Yonhap

Though the North has claimed the test was a successful trial of a hydrogen bomb, experts have expressed skepticism. The White House noted that its intelligence "calls into serious question" the claims of a successful test. Hydrogen bombs are exceptionally powerful weapons with a significantly more powerful blast than the atomic bombs used on Hiroshima and Nagasaki during World War II that killed more than 200,000 people.

Yonhap reported early Thursday that South Korean intelligence officials believed North Korea could be closer to miniaturizing nuclear weapons, typically the final step in creating nuclear warhead-tipped inter-continental ballistic missile.

"It is beyond our intelligence whether North Korea has developed a miniaturized hydrogen bomb, but it is still believed to have achieved much of the miniaturization technology itself, given the time the country has taken in its nuclear development so far," an anonymous intelligence official said, according to the news agency.