SEOUL, South Korea — A municipal worker from Ohio on a tour of North Korea has been detained there for unspecified illegal acts, making him the third American known to be currently held in the isolated country and threatening to further aggravate its tense relationship with the United States.

North Korea’s state-run Korean Central News Agency reported the detention on Friday, saying the tourist, Jeffrey Edward Fowle, had entered North Korea on April 29 and “perpetrated activities that violated the laws of our republic, which did not fit his stated purpose of visiting our republic as a tourist.”

The agency did not explain the nature of the accusations, but Japan’s Kyodo News Agency, citing unidentified sources, said Mr. Fowle had been seized in mid-May as he was about to leave the country and that he had left a Bible in his hotel room, which the North Korean authorities might have interpreted as the subversive work of a missionary.

The Korean Central News Agency provided no further details about Mr. Fowle but local media in the Dayton, Ohio, area said that he was a 56-year-old municipal worker in the suburb of Moraine and had a wife and three children. The website of the Dayton Daily News said the Moraine city manager, David Hicks, had described him as a longtime employee. Telephone messages left on Mr. Fowle’s home telephone answering machine and with Mr. Hicks’s office were not returned.