President Donald Trump has reacted to what he is calling "a major nuclear test" by North Korea, branding North Korea "a rogue nation" whose "words and actions continue to be very hostile and dangerous" to the United States.
Trump said on Sunday "we'll see" when asked by reporters if he planned to attack North Korea.
Trump also rebuked South Korea on Twitter, saying "their talk of appeasement with North Korea won't work, they [North Korea] only understand one thing".
The White House said in a statement the situation was being closely monitored, and Trump and his national security team would meet later in the day.
German Chancellor Angela Merkel and French President Emmanuel Macron called for tougher EU sanctions against North Korea, saying Pyongyang has "reached a new dimension of provocation" with its latest nuclear test.
"The chancellor and the president are in agreement that North Korea has trampled on international law and that the international community must, therefore, react with determination against this new escalation," Merkel's office said in a statement after she spoke on the phone with Macron.
Separately, German Foreign Minister Sigmar Gabriel said Pyongyang's latest test "means that we have to find a level-headed but clear answer".
"We will discuss this reaction with our partners in the EU. I am sure that the UN Security Council will also take necessary measures in a decisive manner," he said.
"The two leaders agreed to stick to the goal of denuclearisation on the Korean Peninsula and keep close communication and coordination to deal with the new situation," Xinhua said in a brief dispatch.
Boris Johnson, UK's foreign minister, said North Korea's latest nuclear test could represent a new order of threat.
"There's no question that this is another provocation, it's reckless," he told Sky News broadcaster.
"They seem to be moving closer towards a hydrogen bomb which, if fitted to a successful missile, would unquestionably present a new order of threat."
Yukiya Amano, the head of the International Atomic Energy Agency (IAEA), called North Korea's sixth nuclear test since 2006 "an extremely regrettable act".
The new test "is in complete disregard of the repeated demands of the international community", said Amano.
The IAEA is continuing to closely follow developments in North Korea's nuclear programme "which is a matter of grave concern", he said.
Jens Stoltenberg, NATO's secretary-general, called on North Korea in a statement to "immediately cease all existing nuclear and ballistic missile activities in a complete, verifiable, and irreversible manner, and re-engage in dialogue with the international community".
South Korea's President Moon Jae-in vowed to push for the most powerful sanctions yet at the UN Security Council against North Korea to completely isolate it.
Shinzo Abe, Japan's prime minister, said Pyongyang's "nuclear and missile development programmes pose a new level of a grave and immediate threat" and "seriously undermines the peace and security of the region".
China's foreign ministry urged North Korea in a statement broadcast by state-run CCTV to "stop taking erroneous actions that deteriorate the situation".
Russia's foreign ministry said in a statement immediate dialogue and negotiations were the only way to settle the Korean Peninsula's problems, "including the nuclear one".
The ministry said Russia reaffirmed its readiness to participate in negotiations, "including in the context of the implementation of the Russian-Chinese road map".
Under that proposal, North Korea would suspend nuclear and missile tests in exchange for the United States and South Korea suspending their joint military exercises.