North Korea's biggest nuclear test to date was condemned around the world on Sunday, with several leaders calling for new sanctions and U.S. President Donald Trump saying "appeasement" would not work.
The explosion of what North Korea said was an advanced hydrogen bomb came just days after it fired a missile over Japan.
Trump, who said after last week's missile launch that talking to Pyongyang "is not the answer", tweeted that Sunday's test showed North Korea's "words and actions continue to be very hostile and dangerous to the United States".
U.S. Treasury Secretary Steve Mnuchin said he would put together new sanctions to potentially cut off all North Korea's global trade.
"This latest provocation by the ruler in Pyongyang has reached a new dimension," the German government said in a statement after Merkel and Macron discussed the issue by phone.
Britain's foreign minister Boris Johnson said: "They (North Korea) seem to be moving closer towards a hydrogen bomb which, if fitted to a successful missile, would unquestionably present a new order of threat," adding that there were no palatable military solutions.
British Prime Minister Theresa May said the U.N. Security Council should urgently look at new measures.
Russia struck a cautious tone.
"In the emerging conditions it is absolutely essential to keep cool, refrain from any actions that could lead to a further escalation of tensions," Russia's foreign ministry said, adding that North Korea risked "serious consequences".
Later on Sunday, Russian President Vladimir Putin met his Chinese counterpart Xi Jinping in China where they agreed to "appropriately deal with" the crisis, Chinese state news agency Xinhua said.
"The two leaders agreed to stick to the goal of denuclearization on the Korean Peninsula and keep close communication and coordination to deal with the new situation," Xinhua said.
Earlier, China urged North Korea to stop "wrong" actions and said it would fully enforce U.N. resolutions on the country.
As North Korea's most important trading partner, the position of China - a permanent member of the U.N. Security Council — will be closely watched. A Japanese government source said there would be pressure on Beijing to impose an oil embargo.
"They will probably act eventually but ... it is possible that will not be before their October (party) convention," the source said. "Russia does not have real influence on North Korea. It's China that matters."
Trump said North Korea had become "a great threat and embarrassment to China" and that Beijing had tried but failed to solve the problem. What he called South Korea's "talk of appeasement" would not work as "they (the North Koreans) only understand one thing!"
The International Atomic Energy Agency, which has no access to North Korea, called the nuclear test, Pyongyang's sixth since 2006, "an extremely regrettable act" that was "in complete disregard of the repeated demands of the international community".