Korean-American tourist Kenneth Bae, also identified as Pae Jun-Ho, was reportedly arrested last month as he entered the north-eastern port city of Rason near North Korea's border with Russia and China.
His custody comes amid tension between Pyongyang and Washington over a recent North Korean rocket launch, which US officials consider a provocative test of ballistic missile technology.
"In the process of investigation, evidence proving that he committed a crime against the DPRK was revealed. He admitted his crime," the state news agency KCNA reported.
KCNA said Swedish Embassy officials had visited Mr Bae on Friday but provided no details of his condition or of the crime he was charged with.
Sweden handles the affairs of US citizens in North Korea because the United States does not have diplomatic relations with North Korea, or the Democratic People's Republic of Korea (DPRK), as it is officially known.
According to North Korean law, the punishment for hostile acts against the state is five to 10 years of hard labour.
The arrest was first reported earlier this month by a South Korean newspaper, Kookmin Ilbo, which had identified the detainee as a 44-year-old Korean-American tour operator.
The newspaper said he had been travelling with five other tourists and was detained when a computer hard disk was found among the group's belongings.
The report, which has not been verified, said it contained footage of North Korea executing defectors and dissidents.
Running into trouble
US citizens of Korean descent have previously run into trouble in the North.
Robert Park, a missionary, was detained after entering the country in late 2009 and says he was tortured for protesting against human rights abuses.
Earlier that year, former US President Bill Clinton flew to Pyongyang to secure the release of two American journalists who had entered North Korea illegally.
The two were sentenced to 12 years of hard labour in a work camp for crossing the border illegally and "committing hostile acts".