ATHENS, Nov. 15 (Xinhua) -- Greek Prime Minister Alexis Tsipras declared Wednesday a day of national mourning as the death toll from the flash floods that hit western Attica region, at the outskirts of Athens, climbed to 15.

"It is a very difficult time for all of Greece. We are experiencing a great disaster...I want to express my deep sorrow, sincere condolences to the families of the victims. And I promise that we will stand by them with all the means in our disposal," he said in a televised message.

In an e-mailed statement, the Greek leader also pledged that nobody will be left homeless and measures will be taken immediately to support households and businesses affected, while authorities will look into the causes of the disaster.

The strong torrential downpour which turned roads into muddy rivers in the municipalities of Mandra, Nea Peramos and Megara, about 20 kilometers west of the Greek capital, carrying away cars and knocking down walls, has claimed 15 lives, according to the latest official count by the Health Ministry.

Ten of the victims were men and five women, while most were elderly people, local authorities said.

Another 17 people were injured, while an unclear number of people has been reported missing. Rescue efforts by the Fire Brigade to locate and free people stranded in flooded areas will continue through the night, according to authorities.

From the early hours of Wednesday until sunset, the Fire Department had received over 600 calls for help and rescued 86 people trapped in their homes and cars.

As a state of emergency has been declared in the area, a prosecutor has launched an investigation into the causes of the disaster. Greek scientists have pointed to climate change and urban planning violations.