Pyongyang has detained a number of US citizens in recent years, using them as a tool to extract visits by high-profile figures, including former President Bill Clinton, for propaganda means.
North Korea periodically accuses the United States of military hostility and conspiracy to overthrow its leadership. The two states have been locked in a tense diplomatic conflict over Pyongyang's nuclear and missile programs.
The latest American to be held was being questioned by authorities for conduct inappropriate for the purpose of his visit as a tourist, state media reported on Friday.
The North's KCNA news agency named him as Jeffrey Edward Fowle and said he entered the country on April 29. It gave no further details.
A State Department official said Washington was aware of reports that a third US citizen had been detained in North Korea. "There is no greater priority for us than the welfare and safety of US citizens abroad," the official said, adding no further information was available.
Earlier on Friday, Japan's Kyodo news agency said the North had detained a US citizen in mid-May.
Kyodo cited unidentified diplomatic sources as saying the American was detained just before he was to leave North Korea, for having left a Bible in his hotel.
Two other Americans are currently being held by the North, arrested after arriving on tourist visas and accused of crimes against the state. Korean American missionary Kenneth Bae has been in custody for 18 months and a second man has been held since April.
In May, the US State Department issued an advisory urging Americans not to travel to North Korea because of the "risk of arbitrary arrest and detention" even while holding valid visas.
"Foreign visitors to North Korea may be arrested, detained, or expelled for activities that would not be considered criminal outside North Korea," it said.
War veteran released
North Korea has detained and then released other Americans in the past year, including Korean War veteran Merrill E. Newman, whom it expelled after holding him for more than a month accusing him of war crimes.
In April, the North said it was holding an American named Matthew Todd Miller who had made "a gross violation of its legal order" after entering the country on a tourist visa.
He tore up his visa and demanded asylum, KCNA said in April.
Bae was arrested in 2012 and has been sentenced to 15 years' hard labor on charges of state subversion. His family says he suffers from a variety of health issues, including diabetes, an enlarged heart, kidney stones and severe back pain.
North Korea has twice canceled visits by Robert King, the US special envoy for North Korean human rights issues, to discuss Bae's case.
The United States has no diplomatic ties with North Korea and the interests of its citizens in the country are represented by Sweden, which has an embassy in Pyongyang.
(Additional reporting by Elaine Lies in Tokyo; Editing by Raju Gopalakrishnan)