Portuguese police are stepping up their efforts to find Madeleine McCann, who has been missing for five days from a holiday apartment in the Algarve.
Police have faced criticism over their handling of the case
Officers said they had quizzed 100 people and searched 500 apartments.
British ambassador to Portugal, John Buck, said police were doing everything they could to find the girl, following some criticism of the investigation.
Three-year-old Madeleine, from Leicestershire, disappeared from a resort in Praia da Luz on Thursday.
Mr Buck said he had met Madeleine's parents Gerry and Kate again on Monday, and was continuing to support consular staff in the Algarve.
He said additional British experts arrived on Monday to work with the Portuguese on the case.
Mr Buck said: "This is, and must remain, a Portuguese police investigation. As you know, the Portuguese police operate under Portuguese law, and Portuguese law puts constraints on what they can say publicly and the information they can release."
He said he had been in close touch with Portuguese cabinet ministers, the prime minister's office and the police.
"They all assure me that everything possible is being done to ensure the safe return of Madeleine," he said.
Mr Buck added: "I, and I know Kate and Gerry, with whom I've just been speaking for the past hour, are very grateful for their efforts."
Portuguese television had reported that a man and a child matching Madeleine's description were seen driving off in a car from the town of Nelas, about 300 miles (483km) north of Praia da Luz.
However, police - who are following up 350 pieces of information - said the sighting was a case of mistaken identity.
Olegario de Sousa, of the Portuguese police, said: "It was not Madeleine. It was a child who looks like Madeleine, but she is a child who lives in the area."
Madeleine has not been seen since Thursday night
He said officers were maintaining a "high level" of effort to find Madeleine, as well as gathering evidence about her disappearance.
The force had been criticised for not keeping the public informed, but officials have insisted that Portuguese law forbids police from releasing information about a live investigation.
Mr Sousa's press conference, where he listed the action taken by his officers, came after intense pressure from the media.
He told assembled journalists than an area with a radius of 15km (10 miles) around the Algarve resort had been searched - along with hundreds of nearby apartments.
Antonio Santana Carlos, the country's UK ambassador, said the case was of "great concern" to Portugal and asked people to trust the police.
Footballers including England captain John Terry and Portuguese players Cristiano Ronaldo and Paulo Ferreira, who both play in England, have appealed for anyone with information to come forward.
A set of pyjamas identical to the ones Madeleine was wearing
Meanwhile, villagers in Rothley, where the McCanns are from, have held a silent vigil to show support for Madeleine.
Staff at Leicester's three hospitals also joined in prayer for the McCann family.
Madeleine's father is a consultant cardiologist at the city's Glenfield Hospital and her mother is a GP.
The McCann family have confirmed she was wearing white pyjama bottoms with a small floral design and a short-sleeved pink top with a picture of Winnie the Pooh character Eeyore when she disappeared.
She is believed to have been taken from her bed while her parents were eating at a nearby restaurant.
They were regularly checking on her and their two-year-old twins, Sean and Amelie.
The complex where Madeleine disappeared