North Korea says it tested hydrogen bomb; Trump denounces its 'hostile' behavior

North Korea's neighbors are looking for radiation from its nuclear test, but they might not find any.

The North said the underground test site where it detonated what it described as a hydrogen bomb did not leak radioactive materials. If that's true, it will be difficult for outsiders to determine whether the device was indeed a thermonuclear weapon or a simpler nuclear bomb.

Japan's Nuclear Regulation Authority says no abnormal change in radiation levels had been detected on monitoring posts across the country as of Sunday night.

China's National Nuclear Safety Administration says it activated nuclear radiation-related environmental contingency plans shortly after the test was conducted. It said in a statement on its website that automatic environmental radiation monitoring stations in China's northeast were operating normally.

Chinese President Xi Jinping and Russian President Vladimir Putin have discussed North Korea's nuclear test in a meeting on the sidelines of a Beijing-led summit of five large emerging economies.

The official Xinhua News Agency said Sunday the two leaders "unanimously agreed to adhere to the goal of the denuclearization of the Korean Peninsula, have close communication and coordination and properly respond" to Sunday's test.

North Korea detonated what it called a hydrogen bomb. It was its sixth and most powerful nuclear test to date.

The blast could overshadow the summit of BRICS nations - Brazil, Russia, India, China and South Africa. It's being held Monday and Tuesday in the southeastern Chinese city of Xiamen.

Xi gave a speech to business representatives of the five countries on Sunday without mentioning the test.

Finland has strongly condemned North Korea's nuclear test, saying it continues that country's "series of grave violations of international obligations, which have become more common during this year."

Describing the test as "a very dangerous and irresponsible act that will further aggravate the situation," Foreign Minister Timo Soini said Sunday that claims the test was the strongest conducted by North Korea so far and that according to its own statement was a hydrogen bomb "are particularly alarming."

In neighboring Sweden, Foreign Minister Margot Wallstrom tweeted that the test was "a turn for the worse. Further endangers international peace, stability. UNSC role important," while Norway's foreign minister, Borge Brende, said North Korea's action was "totally unacceptable" and urged the international community to react.

British Prime Minister Theresa May has said the North Korean weapon test poses an "unacceptable further threat" to the international community.

May said in a statement Sunday that the international community must come together to increase the pressure on North Korea.

She said tougher action is needed, including speeding up the implementation of sanctions.

The head of the U.N. test ban treaty organization says the sanctions against North Korea over its nuclear weapons program aren't working.

Lassina Zerbo, who heads the Vienna-based Comprehensive Test Ban Organization, also notes that while North Korea previously announced its nuclear tests in advance, now the announcements are coming hours after the test.

Zerbo said that international sanctions seem not to be stopping the North "from going beyond the acceptable in terms of their nuclear weapons program."