Seoul/Beijing
South Korea unleashed an ear-splitting propaganda barrage across its border with North Korea on Friday in retaliation for its nuclear test, while the United States called on China to end "business as usual" with its ally.
The broadcasts, in rolling bursts from walls of loudspeakers at 11 locations along the heavily militarised border, blared rhetoric critical of the Pyongyang regime as well as "K-pop" music. North Korea later responded with its own broadcasts.
Wednesday's nuclear test angered both the United States and China, which was not given prior notice, although the U.S. government and weapons experts doubt Pyongyang's claim that the device it set off was a hydrogen bomb.
China is North Korea's main economic and diplomatic backer, although relations between the Cold War allies have cooled in recent years.
China's Foreign Ministry urged North Korea to stick to its denuclearisation pledges and avoid action that would make the situation worse, but also said China did not hold the key to resolving the North Korean nuclear issue.
"Achieving denuclearisation of the Korean Peninsula and safeguarding the peninsula's peace and stability accords with all parties' mutual interests, is the responsibility of all parties, and requires all parties to put forth efforts," ministry spokeswoman Hua Chunying told a news briefing.
The North agreed to end its nuclear programme in international negotiations in 2005 but later walked away from the deal.
U.S. Secretary of State John Kerry said on Thursday that he had told Chinese Foreign Minister Wang Yi that China's approach to North Korea had not succeeded.
"APPROACH HAS NOT WORKED"
"China had a particular approach that it wanted to make, that we agreed and respected to give them space to implement that," Kerry told reporters after the phone call. "Today (on Friday), in my conversation with the Chinese, I made it very clear that has not worked and we cannot continue business as usual."
In a call on Friday with his South Korean counterpart, Yun Byung-se, Wang said talks on the issue should be resumed as soon as possible, China's Foreign Ministry said.
South Korea's nuclear safety agency said it had found a minuscule amount of xenon gas in a sample from off its east coast, which could be the first chemical evidence of a nuclear test, but said more analysis and samples were needed to determine if it came from a nuclear test.
The presence of xenon would not indicate whether the blast was from a hydrogen device or not.
Seismic waves created by the blast were almost identical to those generated in North Korea's last nuclear test in 2013, Jeffrey Park, a seismologist at Yale University, wrote in a post on the Bulletin of the Atomic Scientists website, adding to scepticism about the hydrogen bomb claim.
Meanwhile, South Korea resumed its frontier broadcasts, which the isolated North has in the past threatened to stop with military strikes.
