South Korea has put its military on alert and beefed up surveillance of North Korea Wednesday after the communist country announced it successfully tested a hydrogen bomb.

"Currently, our military is strengthening our vigilance and surveillance posture against North Korea," the Ministry of National Defense said in a brief note to the press.

Earlier in the day, North Korea announced on its state-run broadcasting channel that North Korea's "first hydrogen bomb test was successfully conducted."

The news came about two hours after a 5.1-magnitude earthquake was registered near North Korea's nuclear test site in the northeastern region.

"Regarding the situation, South Korea and the United States are closely cooperating," the ministry said.

In a 10-minute telephone conference with United States Forces Korea commander Gen. Curtis Scaparrotti earlier in the day, Joint Chiefs of Staff Chairman Lee Sun-jin ensured close collaboration with the U.S., a ministry official said.

They plan to have another online meeting later in the day, he said.

The ministry added it has also launched an emergency crisis management body over what could be the North's latest and fourth nuclear test.

Meanwhile, a military expert well versed in the North's nuclear program questioned the validity of the country's claim of the hydrogen bomb test, citing seismic data.

"The scale of the latest nuclear test fell short of that of the third nuclear test (in 2013)," the expert said.

The local meteorological administration put the magnitude of the tremor resulted from the test at 4.8, which is less than the 4.9 magnitude registered in the previous nuclear test three years ago.

The explosive power of a hydrogen bomb is up to 1,000 times stronger than that of an atomic bomb, and North Korea cannot afford a hydrogen bomb test inside the country, the expert stressed.

Even if the tested bomb included hydrogen, it must have been a very low-end bomb, he said. (Yonhap)