An American woman was briefly detained in Bolivia after arriving on an American Airlines flight from Miami with 500 bullets in her luggage, the state news agency reported.

News agency ABI said the woman, who it identified as Din Dona Thin, 20, had become nervous when taken to have her bags searched on arrival late on Wednesday, telling staff she was only carrying cheese.

"Last night at El Alto Airport, a US citizen was intercepted, coming from Miami, on an American Airlines plane, carrying five boxes of bullets, each one containing 100 .45-caliber bullets," ABI quoted national immigrations director Magaly Zegarra as saying.

A Bolivian judge told reporters the American woman had been released on Thursday because no offense had been committed under the country's laws.

Citing the tight security at US airports, Zegarra said it was "incredible that she was able to pass through with luggage containing five boxes of bullets," ABI reported. It said the official had asked the airline for an explanation.

A US Embassy spokeswoman confirmed in a statement that an American woman had been detained. "There was a lack of knowledge about current customs regulations in Bolivia," the statement said.