Police carried out new searches at a reservoir in Portugal on Tuesday connected to the disappearance of British toddler Madeleine McCann, who vanished from a resort in the Algarve region 16 years ago.

Witnesses on the scene of the remote Arade reservoir, around 50 kilometres from the Praia da Luz resort, saw police officers searching the bank along the dam with the help of a sniffer dog.

Two officers searched the water on board a fire department dinghy.

German prosecutors confirmed the search was taking place but said they could not give further details "for tactical investigative reasons".

Portuguese police had announced on Monday evening that new searches would be carried out "in the next few days, at the request of the German authorities" and "in the presence of the English authorities".

The steps were being taken with a view to "a total clarification of the situation", the police said in a statement.

German police are assisting the Portuguese authorities, the German prosecutors said, while London's Metropolitan Police said it also had officers on the ground.

Madeleine McCann was three years old when she went missing from her family's holiday apartment in Praia da Luz in 2007 while her parents dined at a nearby tapas bar.

Despite a huge international manhunt, no trace of her has been found and no one has been charged over her disappearance.

German prosecutors revealed in June 2020 that they were investigating a German man, Christian Brueckner, in connection with the case, saying they had "concrete evidence" he killed Madeleine.

According to police, Brueckner lived in the Algarve region between 1995 and 2007, making a living doing odd jobs as well as burgling hotel rooms and holiday apartments.

Portuguese weekly magazine Expresso has reported that he would regularly spend time near the reservoir outside the small inland town of Silves. (AFP)