Yellow and green balloons are to be released in 50 countries to mark the 50th day since the disappearance of Madeleine McCann.

The event is part of a global campaign spearheaded by the missing girl's parents to promote awareness of their daughter's abduction in the hope that she will be spotted.

Gerry and Kate McCann's international information drive has already yielded reports from members of the public claiming to have seen Madeleine.

Possible sightings of the four-year-old in Malta are the latest claims to be followed up in the search for the missing girl.

Maltese newspaper Nazzjon printed claims that two tourists had reported seeing a girl matching Madeleine's description in the capital Valletta on Saturday.

Later reports said four other people had since come forward with possible sightings on Malta.

Superintendent Pierre Calleja of the Maltese CID confirmed that a "full-scale" police investigation was under way following the reported sighting.

"On June 17, 2007 the Malta police were in receipt of a report of a sighting of a girl who resembled Madeleine McCann who was abducted in Portugal on May 3, 2007," he said.

"Subsequent to this report, a full scale police investigation and a Magisterial Inquiry were initiated.

"These investigations are actively in progress and all possible avenues are being pursued."

Ray Roberts, from Anglesey, north Wales, said he had reported an 'odd' scene in the Maltese town of Sliema in which he saw a man telling a child: "Get up little girl" in accented English.

He added that the girl's hair was very dark and cut in an unusual style, leading him to conclude it was "very much like a wig rather than real hair."

"The more I think about what I saw the more convinced I become that it may well have been Maddie," Mr Roberts said.

A spokeswoman for parents Gerry and Kate McCann say they were not giving credence to the claims at this stage and were instead focusing on events to boost awareness of her disappearance.

She added: "Their whole attitude from day one has been that they know there are going to be lots of bits of information but if they expended energy on every single one of them what would they be like by now?

"Until they hear that something has been checked out and there is a resolution of the piece of information one way or the other and that comes from an official source they are not going to focus on it."