South Korea launches propaganda barrage over nuclear bomb test.

South Korea unleashed a high-decibel propaganda barrage across the border on Friday in retaliation for North Korea's nuclear bomb test this week, while the United States called on China to end "business as usual" with its ally Pyongyang.

The broadcasts, in rolling bursts from walls of loudspeakers at 11 locations along the heavily militarized border, blare rhetoric critical of the Pyongyang regime as well as "K-pop" music. The broadcasts, considered an insult by the North, led to an exchange of artillery fire the last time they were used.

South Korea, which has grown increasingly close to China in recent years, also said its foreign minister would speak with his Chinese counterpart later on Friday.

Wednesday's 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 exploded was a hydrogen bomb.

U.S. Secretary of State John Kerry said on Thursday he had made clear in a phone call with Chinese Foreign Minister Wang Yi that China's approach to North Korea has not succeeded.

"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. "Today in my conversation with the Chinese I made it very clear that has not worked and we cannot continue business as usual."

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

China's foreign ministry said after the call with Kerry that Beijing was willing to communicate with all parties, including the United States.

"Wang Yi stressed that China has staunchly dedicated itself to the goal of the peninsula's denuclearization and to maintaining peace and stability on the peninsula," the Chinese Foreign Ministry said in a short statement.

South Korea's foreign ministry had requested a phone call with Wang since directly after North Korea announced on Wednesday it had tested a hydrogen bomb, the South's Yonhap News Agency said. However, the call had been delayed due to China's "internal scheduling", it said, citing an unnamed official.

TROOPS DEPLOYED, TOURS CANCELLED

The last time Seoul deployed the loudspeakers, in retaliation for a landmine blast in August that wounded two South Korean soldiers, it led to an armed standoff and exchange of artillery fire.

The sound from the speakers is so loud that it can carry for 10 km (6 miles) into North Korea during the day and more than twice that distance at night, Yonhap reported.

North Korea boosted troop deployments in front-line units on Friday, and South Korea heightened military readiness to its highest level at locations near the loudspeakers.

Seoul also vowed to retaliate against any attack on the equipment and raised its cyber security alert level.

Tours of the Demilitarised Zone, popular with visitors to South Korea, were also canceled at the military's request.

The vast majority of North Korea's business dealings are with China, which bought 90 percent of the isolated country's exports in 2013, according to data compiled by South Korea's International Trade Association.

Kerry said he and Wang agreed to work closely to determine what measures could be taken given increasing concerns about the nuclear test. He said America has a "firm and continued commitment to regional security and global nonproliferation".

The Global Times, an influential Chinese tabloid published by the ruling Communist Party's official People's Daily, said in an editorial it was unfair to expect China alone to bring about change in Pyongyang.

"There is no hope to put an end to the North Korean nuclear conundrum if the U.S., South Korea and Japan do not change their policies toward Pyongyang. Solely depending on Beijing's pressure to force the North to give up its nuclear plan is an illusion," it said.

"The China-North Korea relationship should not be dragged into antagonism. Beijing has participated in previous sanctions on the North. Whether China will take tougher measures hinges on the decision of the UN Security Council," it said.

U.S. Republicans and Democrats in the House of Representatives could join forces in a rare display of unity to further tighten sanctions on North Korea.

Nancy Pelosi, the House Democratic leader, told reporters that Democrats would support a North Korea bill likely to be brought for a vote by Republicans next week. A congressional source said it was expected as soon as Monday.

The House measure would target banks facilitating North Korea's nuclear program and authorize freezing of U.S. assets of those directly linked to illicit North Korean activities. It would also penalize those involved in business providing North Korea with hard currency.

It was unclear how more sanctions would deter North Korea, which has conducted four nuclear tests since 2006 while paying little heed to international pressure.

The United States and its ally South Korea are limited in their military response. Washington sent a pair of nuclear-capable B-2 stealth bombers on a sortie over South Korea in a show of force after North Korea last tested a nuclear device in 2013.

North Korea responded then by threatening a nuclear strike on the United States.

A South Korean military official told Reuters that Seoul and Washington had discussed the deployment of U.S. strategic assets on the divided Korean peninsula, but declined to give further details. Media reports said those assets could include B-2 and B-52 bombers, and a nuclear-powered submarine.