"They include children from the United States, Canada, Poland and France," said Panagiotis Pardalis of the Smile of the Child charity.
In a case that has generated huge interest in Greece, authorities have charged the Roma couple with abducting the child they call "Maria." They appeared in court Monday and were remanded into custody pending a trial.
A lawyer for the couple says the pair adopted the child from her biological mother.
The Smile of the Child said the girl, who was found Thursday in a Roma community near Larissa, central Greece, is now being cared for in a group home.
Medical tests carried out on the girl since she was found indicate she is between 5 and 6 years old, slightly older than initially thought, said Pardalis.
Police have said they are suspicious of the records the couple provided for the child and for other children in their care. In addition to the abduction charge, the couple is accused of falsifying official documents.
Four officials, including the head of the registry office from which Maria got her birth certificate, have been suspended while a police investigation is under way, the media office of the Athens municipality said Tuesday.
The girl received the document this year, it said. It is unusual for a birth certificate to be issued years later.
1,000 years of Roma discrimination
Authorities asked questions about Maria because she has fair skin and blond hair, while her parents have darker complexions typical of Roma, a race descended from Indian nomads who face widespread discrimination in Europe.
Haralambos Dimitriou, head of the local Roma community, said the couple took the girl in because her Bulgarian mother couldn't keep her. He said Maria was raised like a "normal" child.
Pardalis said Sunday that she was found in "bad living conditions, poor hygiene."
Calls about the girl
Thousands of calls poured into Greece after authorities released photos of the girl last week.
Deborah Bradley and Jeremy Irwin, whose daughter Lisa Irwin vanished from their home in Kansas City, Missouri, two years ago aged about 11 months, asked the FBI to get in touch with Greek authorities when they heard about the case.
"There is no such thing as a tip too small," said Bradley, whose hopes were raised despite the apparent disparity in age between their missing daughter and the girl found in Greece.
Authorities released photos of the two adults charged in the case Monday -- Eleftheria Dimopoulou, age 40, and 39-year-old Christos Salis -- in the hope that the publicity would reach someone who can provide more information about them.
Police said the blond child looked nothing like the man and woman with her, and DNA testing confirmed that they were not her biological parents.
A police statement said the couple "changed repeatedly their story about how they got the child."
A government news agency said police found suspicious birth and baptism records as well as family registrations that claimed the woman had given birth to 10 children and the man was the father of four more.
Prejudice against the Roma
Prejudice and discrimination against the Roma are widespread in Greece and elsewhere in Europe, Amnesty International says.
Maria's case plays into old prejudices about them stealing children for forced labor.
Pardalis mentioned such a possibility, saying, "We don't have any other information if this girl was forced to work or to beg on streets."
And the government news agency raised "the possibility of the existence of a ring bringing pregnant women to Greece from Bulgaria and then taking their children for sale." The agency also cited past "reports" that empty coffins were found for infants who supposedly were stillborn to foreign mothers in Athens.
Photo blog: The plight of the Roma