A day after North Korea claimed it had tested a hydrogen bomb, the United States flew its air force's reconnaissance planes from its base in Japan on Thursday, probably to investigate the claims.
Three intelligence-gathering planes, including the RC-135S, which collects data from ballistic targets, took off from the Kadena Air Base in Okinawa in Japan, The Associated Press reported.
While the mission of the US aircraft was not clear, it is speculated to be the next step in investigating North Korea's claims after White House spokesman Josh Earnest said on Wednesday the US would be "collecting additional evidence."
White House officials had been sceptical of the Kim Jong-un regime's claims, stating that initial data was "not consistent" with the testing of a hydrogen bomb.
North Korea made an announcement on Wednesday morning after an artificial earthquake was recorded near the Punggye-ri nuclear site in the country.
If confirmed, this would be the first time a hydrogen bomb has been successfully tested in the isolated country.
The United Nations Security Council, which held an urgent meeting on Wednesday, has threatened to take action against the Kim Jong-un regime.
South Korea, neighbour and arch-rival of the North regime, also expressed scepticism over the testing.
While the claims of a hydrogen bomb are being dismissed, US and South Korean officials have said North Korea is reaching the last stages of developing miniaturised nuclear warheads.