Bomb test sees heat turned up on China

The United States has called on China to end ‘‘business as usual’’ with its ally North Korea, after Pyongyang defied world powers by announcing it had tested a hydrogen bomb.

US Secretary of State John Kerry said he made it clear in a phone call with Chinese Foreign Minister Wang Yi yesterday 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 said. ‘‘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. The vast majority of North Korea’s business dealings are with China.

North Korea carried out a nuclear test on Thursday, although the US government and weapons experts doubt Pyongyang’s assertion that the device it exploded was a powerful hydrogen bomb.

The test angered both the US and China, which was not given prior notice.

Kerry said he and Wang agreed to work closely to determine what measures could be taken, given increasing concerns about the nuclear test.

‘‘Sniffer’’ planes and other sensors had yet to detect any evidence, such as particles in the air, that would substantiate the North Korean assertion that it had set off an H-bomb, a US government source said.

US 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, said Democrats would support a bill likely to be brought for a vote by Republicans next week. It would target banks facilitating North Korea’s nuclear programme and authorise the freezing of the US assets of those directly linked to illicit North Korean activities. It would also penalise 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.

A South Korean military official said Seoul and Washington had discussed the deployment of US strategic assets on the divided Korean peninsula, but declined to give further details.

South Korea resumed propaganda broadcasts by loudspeaker into North Korea yesterday, believed to be the birthday of North Korean leader Kim Jong Un.