The United States called on China to put pressure on North Korea to end its nuclear programme after Pongyang conducted its fifth and biggest nuclear test.
US Defense Secretary Ash Carter, on a visit to Norway, also reaffirmed an "iron-clad" commitment to defend South Korea and condemned the North Korean test on Friday as a direct challenge to the entire international community.
Mr Carter said the international community should redouble pressure on North Korea after the test, on the 68th anniversary of North Korea's founding, and singled out China's influence.
"It's China's responsibility," he told a news conference. "China has and shares an important responsibility for this development and has an important responsibility to reverse it."
"And so it's important that it (China) use its location, its history and its influence to further the denuclearisation of the Korean peninsula," he said.
Under 32-year-old leader Kim Jong Un, North Korea has accelerated the development of its nuclear and missile programmes, despite UN sanctions that were tightened in March and have further isolated the impoverished country.
Mr Carter said the nuclear test would strengthen resolve to bolster defences in South Korea, including deployment of US THAAD anti-missile units and radars.
He said he spoke with South Korean Defence Minister Han Min-koo after the test and "reaffirmed our iron-clad commitment to the defence of South Korea and our other allies in the region".
Meannwhile United Nations Secretary-General Ban Ki-moon has also condemned North Korea's nuclear test as a "brazen breach" of UN Security Council resolutions.
"I count on the Security Council to remain united and take appropriate action. We must urgently break this accelerating spiral of escalation," he told reporters.
The UN Security Council is meeting on Friday to discuss the tests.