WASHINGTON (REUTERS) - The United States called on China on Thursday to end its "business as usual" approach 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 clear in a phone call with Chinese Foreign Minister Wang Yi that China's approach to North Korea had failed.

"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.

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

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

The test angered China, which was not given prior notice.

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, 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 legislation was passed by the House Foreign Affairs Committee last February but it was stalled until this week when Pyongyang jolted the world by setting off an underground nuclear bomb test.

The House measure would target banks facilitating North Korea's nuclear programme and authorise freezing of 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.

"We understand Republican leadership plans to move a Bill strengthening US sanctions on North Korea. That will have strong bipartisan support," Pelosi said, adding that "we will support it."

While it is unclear how more sanctions would deter North Korea, which has paid little heed to international pressure as it conducted four nuclear tests since 2006, the United States and its ally South Korea are limited in their military response.

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

A White House spokesman said there had been no talk with South Korea about any introduction of the so-called Terminal High Altitude Area Defence system, a move opposed by China.

"There have been no discussions or consultations with the South Koreans" about the deployment of anti-ballistic missile capability," spokesman Josh Earnest said.

The system has radars that can track multiple ballistic missiles up to 2,000km away, a range which would reach deep into China.