Thai police said on Monday two Americans suspected of trying to send infant and adult body parts in parcels to the United States had fled the country.

A baby's head, a baby's foot sliced into three parts, a heart and a "sheet of skin" with tattoo markings were found in parcels on Saturday after staff at a shipping office in Bangkok scanned the packages, police said.

The parts were stored in plastic containers filled with formaldehyde and the packages were destined for an address in Las Vegas.

"X-rays showed there were contents similar to human body parts. From our investigation of three parcels we found human body parts in five plastic containers," Police Lieutenant General Ruangsak Jaritake, assistant to the National Police Commissioner, told reporters.

"The packages were marked 'children's toys,' but x-rays showed they were not children's toys."

Police named the two suspects, aged 31 and 33, and said they were being "monitored", but did not say how.

Both men fled Thailand on Sunday through a checkpoint in the east of the country, Ruangsak said.

He said the heart had stab marks and belonged to an adult while the sheet of skin with tattoo markings also belonged to an adult.

Museum of Death

"As soon as we have results, we will contact the FBI," he said.

Doctors at Bangkok's Siriraj Hospital said that the body parts were taken from the hospital's Medical Museum, nicknamed the Museum of Death, which exhibits preserved human remains, many of them from murder victims.

"We can confirm that they were stolen from Siriraj," Udom Kachintorn, Dean of the Faculty of Medicine Siriraj Hospital, told reporters.

He added that the two Americans visited the museum last month.

In 2012, Thai police arrested a British citizen of Taiwanese origin after discovering six human fetuses which had been roasted and covered in gold leaf stuffed into travel bags at a hotel room in Bangkok's Chinatown.

Thai detectives said they believed the corpses were due to be sent to Taiwan to be used as part of a black magic ritual.