"A person with a Swedish passport has been arrested. We are trying to figure out who he is," foreign ministry spokesman Andre Mkandawire told AFP.
"We have received information to that effect but the embassy is trying to figure out if the information is correct," he added.
Thailand's Deputy Prime Minister Chalerm Yubamrung had told AFP by telephone Friday that the suspect detained on Thursday was from Lebanon.
"We already have one suspect in custody for interrogation at a government building in Bangkok. He is a Hezbollah from Lebanon," he said.
Mkandawire said Swedish embassy officials were trying to work out the confusion.
"It could be that he has dual citizenship, or it may be that he is a Swede of foreign origin," he explained.
Embassy officials "hope to be able to meet with him over the weekend but that may not happen until Monday," Mkandawire said, adding that the matter was "sensitive."
A senior Thai intelligence officer who did not want to be named told AFP that the kingdom had been informed before the New Year by Israel of a possible threat.