South Korea has unleashed an ear-splitting propaganda barrage across its border with North Korea in retaliation for its nuclear test, while the United States has 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 US and China, which was not given prior notice, although Washington 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 program in international negotiations in 2005 but later walked away from the deal.
US 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.
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 but said more analysis and samples were needed to determine if it came from a nuclear test.
The head of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), which uses monitoring stations around the world to detect atomic tests, said only "normal" levels of xenon had been detected, at a site in Japan.
"Xenon readings at 1st station downwind of #DPRK test site RN38 Takasaki #Japan at normal concentrations. Sampling continues," the CTBTO's executive secretary, Lassina Zerbo, said on Twitter on Friday evening.
The presence of xenon would not indicate whether the blast was from a hydrogen device or a simpler fission explosion.
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.
The last time South Korea deployed the loudspeakers, in retaliation for a landmine blast in August that wounded two South Korean soldiers, it led to an exchange of artillery fire.
The sound can carry 10 kilometres into North Korea during the day and more than twice that at night, the South's Yonhap news agency reported.
A male announcer could be heard from South Korea telling North Koreans that their leader Kim Jong Un and his wife wear clothes costing thousands of dollars.
Another message said Kim's promises to boost both the economy and the nuclear program were unrealistic.