The secretive state revealed it had detained another American, bringing the total number of US citizens it is holding to four.
A North Korea Foreign Ministry spokesman said: “The recently-detained Americans are under investigation of a relevant legislative organ for their criminal acts against the DPRK.
“It is an exercise of the legitimate right of a sovereign state to deal with the criminals according to its law.”
North Korea’s KCNA news agency said that the government would announce shortly the crimes the Americans are accused of.
The statement said: “It will be soon brought to light what kind of crimes the detained had committed,” vowing to “ruthlessly punish the criminals.”
North Korea, or the Democratic People’s Republic of Korea (DPRK), said it had detained Kim Hak Song
Officials in the Communist state said he had been held on suspicion of “hostile acts”.
A third US citizen, Kim Sang Dok, who was associated with the same school, was detained in late April for alleged hostile acts, according to North’s official media.
Out of 10 US citizens detained by North Korea since 2009, two others remain under arrest – Korean-American Christian pastor Kim Tong Chul and college student Otto Warmbier.
Mr Warmbier was sentenced to 15 years hard labour for stealing a banner from a Pyongyang hotel, while Mr Kim was sentenced to 10 years of hard labour on charges of espionage and subversion.
A US State Department spokesperson said: “The security of US citizens is one of the department’s highest priorities.
“When a US citizen is reported to be detained in North Korea, we work with the Swedish embassy in Pyongyang.”
The arrests come at a time of heightened tensions between the two countries with the situation bordering on an all-out military conflict.
Speculation amongst some sections of the US media have speculated that Pyongyang simply “kidnapped” Americans to use them as a bargaining chip for negotiations with Washington.
A Pyongyang spokesman, in a message carried by KCNA, refuted such “impure rumours”, stressing that the “US and other hostile forces are ceaselessly and frantically committing hideous crimes to ‘remove’ the supreme leadership of the DPRK and overthrow its state”.