He has also not been officially identified although he is widely believed to be Robert Park, an American missionary who South Korean activists say crossed over a frozen river into North Korea several days earlier to raise the issue of human rights in the isolated country.
Officials at the US State Department have said they are still seeking consular access to him.
The US has no diplomatic relations with North Korea and any access would be via the Swedish embassy in Pyongyang which represents US interests.
Jo Sung-rae, a South Korean activist who has been the source of most information about Park said on Thursday that he had no knowledge of the second American detainee.
Artillery fire resumes
The arrest comes as North Korean forces resumed firing artillery near the country's disputed western sea border with South Korea.
On Wednesday, North Korea fired about 30 artillery rounds into the sea from its western coast, triggering a brief exchange of fire with a nearby South Korean marine base.
Pyongyang has said the firing is part of ongoing military exercises.
On Thursday, the North fired several artillery shells that are believed to have landed in South Korean waters, an official at Seoul's Joint Chiefs of Staff told the Associated Press.
The official also said that South Korea did not respond but was closely watching the North's actions.
No-one was injured in Wednesday's exchange of fire, although with tensions high on the Korean peninsula such incidents have the potential to escalate rapidly into a dangerous confrontation.
The poorly marked sea border has been a constant source of friction since it was drawn by United Nations forces at the end of the 1950-53 Korean War. The North insists it should be drawn further to the south.
North and South Korean navies fought a skirmish in November that left one North Korean sailor dead and three others wounded, and engaged in bloodier battles in the
area in 1999 and 2002.
Wednesday's exchange was the first between the two Koreas since November's skirmish, and could be aimed at raising tensions to emphasise that the peninsula remains a war zone and push for a treaty formally ending the 1950-53 Korean War.
South Korea and the United States have insisted that North Korea return to
nuclear disarmament talks before any treaty can be concluded.