The focus of police efforts to find missing Madeleine McCann has shifted to South America after an anonymous caller claiming to have knowledge of the four-year-old was traced there.

It is understood the "credible" call was made from a mobile phone registered in Argentina. It was considered so potentially significant that Kate and Gerry McCann, Madeleine's parents, halted their search of Europe to help police investigate.

They delayed their flight from Berlin to Amsterdam by three hours and plans were drawn up to divert to the UK. It was thought the McCanns might need to return to Britain to talk to specialist advisers about the call.

The call from the pay-as-you-go phone came from a man who wanted to speak directly to the McCanns, according to Spanish police sources. He did not reveal his identity or nationality, but the phone was soon linked to the South American country.

All efforts to re-establish contact with the caller failed on Wednesday and the couple carried on with their journey around Europe.

A British police source said: "The importance of this line of inquiry is still being assessed and attempts to re-establish contact are continuing."

Although Spanish officials denied they had received the call, a Guardia Civil source told the Portuguese newspaper Correio de Manha: "Only time will tell if this call gives help or not to the case."

Spanish newspaper El Mundo reported that a man matching the description released by Portuguese police two weeks ago was seen in a bar in Seville a week before Madeleine's abduction.

It claimed the man was working on the instruction of others and told fellow drinkers he was going to the Algarve.

In Praia da Luz, Mr and Mrs McCann planned a trip to Morocco as part of their attempt to raise awareness of Madeleine's disappearance.

They are keen to go because of its proximity to Portugal. Several ferries a day leave from Tarifa in Spain for the 35-minute crossing to Tangiers. The couple are considering catching the ferry and travelling to the capital Rabat.

Morocco has surfaced repeatedly in the hunt for Madeleine, with a reported sighting of a little girl seen with a man.

Yesterday, the McCanns enjoyed a quiet day in Praia da Luz after a hectic series of visits to Italy, Spain, Germany and the Netherlands. At the beach they watched as 1000 yellow balloons calling for information about Madeleine were released into the air.

Balloons were also released in Madrid, London and Manchester.

Earlier, they were the guests of honour at a concert in neighbouring Lagos held in honour of Madeleine, who has been missing for 36 days.

In a separate development, Madeleine's aunt Philomena McCann told yesterday how armed police officers stopped her putting up posters of the little girl at Lisbon airport. She and another relative were travelling from the Algarve to the holy shrine at Fatima when they made a diversion to the airport.

Kate McCann had noticed there were no pictures up when she passed through on her way to Madrid.

"She was so upset to think there were so many tourists coming in and out and nothing there to remind people of Madeleine," said Ms McCann.

"She asked me to make a detour on the way. I was given permission to put the posters up by a woman on the information desk.

"But straightaway we were swooped on by two armed police officers. I was with a relative who was manhandled by them. We went back to the information desk and there was a big row between the woman and the police."

Ms McCann said the director of the airport, Dr Francisco Severino, told them they could fax a request which would be considered.