A protester burns banners depicting North Korean leader Kim Jong Un during an anti-North Korea rally in central Seoul, South Korea, January 7, 2016.

U.S. Secretary of State John Kerry urged China to take a tougher line with North Korea after its latest nuclear test, warning in a call to his Beijing counterpart Thursday that it cannot be “business as usual.”

Rejecting criticism that the U.S. had neglected the North Korean threat while focused on halting Iran’s nuclear program, Kerry said China – Pyongyang’s sole major ally – had taken the lead on the issue.

China’s cautious approach to its isolated and unpredictable totalitarian neighbor has not borne fruit, however, and now Washington is pushing for stronger international action.

“Let me make it clear: North Korea has never been left unattended to. Not for one day,” Kerry told reporters.

“Now China had a particular approach that it wanted to make and we agreed and give them time to implement that,” he said, describing his call to China’s foreign minister Wang Yi.

“But today in my conversation with the Chinese I made it very clear that that has not worked and we cannot continue business as usual.”

North Korea has carried out four nuclear tests since 2006, including this week’s, which it claimed – to international skepticism – was of its first hydrogen bomb.

China joined the global chorus of outrage but has not yet said whether it will impose tough new sanctions.

After Kerry’s comments, a U.S. official admitted that “China has an influence on North Korea that nobody else has.”

Washington has said it will not accept North Korea as a nuclear-armed state and will continue to pile on diplomatic and economic pressure until Pyongyang backs down.

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