South Korea's military said Friday it believed the North had conducted its "most powerful" nuclear test to date, Yonhap news agency said.

"We believe that the North conducted a nuclear test today," it quoted an unidentified defence ministry official as saying.

"The blast is believed to be about 10 kilotons -- the most powerful one by the North so far," the official was quoted as saying.

The North's third nuclear test, staged in February 2013, was previously considered the most powerful to date, with a yield of six to nine kilotons.

South Korea is planning to arrange telephone conferences with the top diplomats of the United States and Japan to discuss North Korea's nuclear test, a ministry official said Friday.

"Foreign Minister Yun Byung-se, now traveling in Laos with President Park Geun-hye, is seeking to have consultations with close allies," the high-ranking official said. 

He noted that Yun is planning to have talks with his U.S. and Japanese counterparts, John Kerry and Fumoi Kishida, over the phone to discuss the issue.

South Korea's military has all but confirmed North Korea conducted its fifth nuclear test. The government's weather service said a magnitude 5 quake was detected with the military saying this tremor could have been caused by a 10 kiloton nuclear device. 

The ministry official said South Korea is also in the process of arranging talks among the top negotiators of the six-party forum on denuclearizing North Korea, a gathering of South Korea, North Korea, the U.S., Japan, China and Russia.

The White House said it is aware of seismic activity near North Korea's nuclear site as the communist nation is strongly suspected of conducting what would be its fifth nuclear test.

"We are aware of seismic activity on the Korean Peninsula in the vicinity of a known North Korean nuclear test site. We are monitoring and continuing to assess the situation in close coordination with our regional partners," National Security Council spokesman Ned Price said.

He did not provide further details.

South Korea said a magnitude 5 artificial earthquake struck near North Korea's Punggye-ri nuclear test site, strongly suggesting the possibility of a nuclear test.

If confirmed, it would be the North's fifth nuclear test. The four previous tests came in 2006, 2009, 2013 and in January this year. (From news reports)