"If this test is confirmed then it will be a new step by Pyongyang on the path of developing nuclear weapons, which is a flagrant violation of international law and existing UN Security Council resolutions," Russia's foreign ministry said in a statement.
"Such actions are fraught with the possibility of aggravating the situation on the Korean peninsula, which already has a very high potential for military and political confrontation," it said.
Moscow called on all sides to "show maximum restraint and to refrain from any actions that could lead to an uncontrolled increase in tensions".
A spokesman for President Vladimir Putin said Moscow was "extremely concerned" by the claims of a hydrogen bomb test by North Korea -- which borders Russia -- and that the Kremlin strongman had told scientists to look into it.
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"President Vladimir Putin has ordered all data from monitoring stations, including seismological stations, to be studied thoroughly and to analyse the situation if a test is confirmed," spokesman Dmitry Peskov was quoted as saying by Russian news agencies.
The claimed test, which came just two days before Kim Jong-Un's birthday, was initially detected as a 5.1-magnitude tremor at the North's main Punggye-ri nuclear test site in the northeast of the country.