Japanese prime minister Shinzo Abe on Sunday called North Korea's sixth nuclear test "absolutely intolerable", but there are conflicting signals from his government about how to respond.
Some in the cabinet, including the prime minister himself, are known to favour pushing the United Nations to impose an international oil embargo on the North.
A full ban on oil exports would trigger a belligerent response from Pyongyang and would probably be blocked by Russia and China, which are permanent members of the UN Security Council.
The government's top spokesman, Yoshihide Suga, said last Wednesday that an embargo – considered a last economic resort – is "one of the options" on the table.
But foreign minister Taro Kono said it "would be close to the last of our options".
Japan is a staunch American ally and hosts dozens of US military bases and thousands of troops, making it a potential target of North Korean aggression, should war start.
Television schedules were interrupted on Sunday to bring news of the explosion, which experts said was much more powerful than the nuclear bomb that destroyed Hiroshima in August 1945.
Defence minister Itsunori Onodera said so-called "sniffer planes", designed to detect radioactivity, had been dispatched to check fallout from the test.
A statement from the prime minister's office condemned Sunday's test and a series of missile launches by the North this year, including Tuesday's ICBM-class missile, which flew over Japanese territory.
Mr Abe said he and US president Donald Trump had agreed to "co-operate closely with the international community, to increase pressure on North Korea and make it change its policies."
Yet, the language, and the condemnations are almost identical to previous statements from the prime minister, underlining Japan’s utter failure so far to deter its troublesome neighbour.
Mr Abe is hoping for support for stronger measures when his party, the ruling Liberal Democrats, discuss the crisis with the opposition in a special parliamentary session on Tuesday.