According to the Red Cross, 26 bodies were found in the yard of a villa in the seaside village of Mati, which is at the centre of the disaster.
Before news of the grim discovery, the official death toll stood at 24.
Emergency workers have used boats and helicopters to evacuate people from a beach.
The authorities are seeking international assistance.
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09:28
Forest fires raging across Greece have killed at least 20 people, the government says, as the authorities there seek international assistance. Hundreds of firefighters are battling the blazes and people have left their homes near the capital, Athens.
A search and rescue operation is underway for 10 tourists who fled one of the fires in a boat, officials say.
"We will do whatever is humanly possible to control it," Prime Minister Alexis Tsipras told reporters.
He cut short an official trip to Bosnia in order to help coordinate the response to the situation, which firefighters have described as "extremely difficult".
Early on Tuesday, government spokesman Dimitris Tzanakopoulos confirmed that the death toll had risen to 20.
Most of the victims had been trapped in the seaside resort of Mati, 40km (25 miles) north-east of Athens, and died either in their homes or their cars.
More than 104 are injured, 11 seriously, he added, with 16 children among the casualties.
Miltiadis Vyronas, from the country's Ekav emergency service, said that 25 people had been taken to hospital and four were seriously injured.
Dramatic video posted on social media showed damaged buildings, orange smoke-filled skies, and people fleeing the fires by car.
Residents in the worst-affected areas were forced into the sea to escape the flames.
"Thankfully the sea was there and we went into the sea, because the flames were chasing us all the way to the water. It burned our backs and we dived into the water... I said my God, we must run to save ourselves," Kostas Laganos in the coastal resort of Mati said.
Prime Minister Tsipras said that "all emergency forces have been mobilised" and declared a state of emergency in the Attica region around Athens.
The government has asked other European countries for helicopters and additional firefighters to help tackle the fires.
Earlier on Monday, residents of a coastal region near Athens were told to abandon their homes as the uncontrolled fires spread. Hundreds of children were also evacuated from holiday camps.
The extreme weather comes as dozens of wildfires continue to rage in Sweden.
Countries including Italy, Germany, Poland and France have all sent help in the form of additional planes, vehicles and firefighters.
But with temperatures set to soar again, they are in a race against time to get the fires under control.
The wildfires are the worst to hit Greece since 2007, when dozens of people were killed in the southern Peloponnese peninsula.
