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's nuclear testing will help restore Japan-South Korean military cooperation".

On Wednesday, Pyongyang claimed it had carried out its first hydrogen bomb test.

China is North Korea's main economic and diplomatic backer, although relations between the two Cold War allies have cooled in recent years.

This week's atomic weapons test by North Korea fueled a round of condemnation by the USA against them, and also against China, with Secretary of State John Kerry declaring that China's policies toward the north had failed and it was time for new strategy.

"China had a particular approach that it wanted to make and we agreed and respected to give them the space to be able to implement that", Kerry told reporters on Thursday.

Chinese officials today dismissed that, saying that the US-led efforts to isolate North Korea have only made matters worse, and that China shouldn't be expected to solve the entire problem by themselves. "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 not. But South Korea sees K-pop and propaganda as quick ways to show its displeasure - and deliver a guaranteed irritant to the North's sensitive and proud leadership.

The South Korean broadcasts are considered an insult by the isolated North which has in the past threatened military strikes to stop them.

South Korean soldiers removing the camouflage as the loudspeakers were put to work again at the border yesterday.

Officials say broadcasts from the South's loudspeakers can travel about 6 miles during the day and 15 miles at night.

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.

The South's Yonhap news agency said Seoul had deployed missiles, artillery and other weapons systems near the border to swiftly deal with any possible North Korean provocation. The observatory looks across the demilitarized zone.

"Continuing with words is not enough, we have to show we are prepared to take actions to ensure sanctions against North Korea are effective", Hammond said.

Beijing's supplying of food and fuel to North Korea has watered down the effect of global sanctions.

However, it should be noted that the Security Council's warnings and sanctions included in its past resolutions concerning North Korea's nuclear and missile programs have been ineffective.

United States President Barack Obama, in a phone call to South Korean President Park Geun Hye yesterday, reaffirmed the US' "unshakeable" commitment to South Korea's security.

US Navy Vice Admiral Joseph Aucoin, commander of the 7th Fleet based in Yokosuka, Japan, says the Navy is watching North Korea closely after the country conducted its fourth nuclear test.

North Korea didn't immediately react, but its response could be especially harsh because of the high emotions surrounding the likely birthday of Kim, who is believed to be in his early 30s.