Greece was in mourning Thursday as rescue crews tried to locate several people missing in a flood that killed 16 people near the capital, with more thunderstorms forecast until the weekend.

Authorities said at least four people were still unaccounted for in Mandra, one of three towns about 50 kilometres (30 miles) west of Athens hit by a freak flood early Wednesday.

The latest victim, a 50-year-old man, was found in a mud-filled basement. It took rescue crews over a day to reach his home.

The poor weather is set to continue until the weekend, raising concerns for hundreds of people with waterlogged homes.

Late on Thursday, the capital was lashed by another thunderstorm and firefighters in northern Greece said they were called to drain water from over 400 homes.

Prime Minister Alexis Tsipras, who declared three days of national mourning after the disaster, said he felt "shock" after touring the area Thursday.

"This is clearly a rare and extreme weather phenomenon," Tsipras said in a statement.

"But this extreme phenomenon had these effects because of (decades of) accumulated problems and deficiencies in infrastructure and zone planning," he said.

Experts have said ill-conceived building in the area -- some of it by local municipal authorities -- meant this was a disaster waiting to happen.

Corrective drainage works for the area were approved in 2016 but work has yet to begin.

Meteorologists said Wednesday's heavy rainfall was concentrated on a nearby mountain that had been devastated by wildfires in 2016, facilitating the ensuing mudslide.

Neighbouring areas saw much less rain, they said.

"It was like a tsunami," Evangelos Kolovetzos, a local shopowner, told AFP.

Local resident Spyros Karambikas told ERT television that he saw a man being swept away by the torrent "like the wind blows away a napkin."

"The water in my house rose to 3.5 metres (11.5 feet)," said Sotiris Loukopoulos, whose pharmacy is the only one still open in Mandra.

"Five pharmacies were destroyed, we are still operating because we are on higher ground," he told Athens municipal radio, as residents tried to clean their yards with shovels and hoses.

Over a hundred firefighters aided by army machinery were mounting search and rescue efforts in Mandra, Nea Peramos and Megara, the semi-rural communities west of Athens hit hardest by the deluge.

The operation unfolded alongside gutted, debris-strewn streets, overturned cars and hundreds of flooded homes and shops as utility crews laboured to restore power and water services.

Emergency crews used pumps to drain water as police reinforcements were sent to the area to prevent looting.

Four people were arrested on Thursday after allegedly attempting to steal electrical appliances from a factory, state agency ANA said.

Twelve people are hospitalised, one in serious condition.

- Illegal buildings -

As a first step, the state will cover the funeral expenses, the interior ministry said.

Food, water and blankets have been rushed to the area, hit by what locals have described as the worst flooding in 20 years.

Some elderly people died inside their homes while other people were trapped in their cars as they drove to work. Two bodies were found at sea.

Parts of the area are without water and electricity for a second day, and much of the damage will take days to repair, though fortunately the sewage system is still functioning, the state water company said.

A 364-cabin cruise ship has been commissioned to shelter some of the homeless if necessary, the merchant marine ministry said.

Once a rural area, Mandra and neighbouring towns were rapidly transformed into a logistics hub for factories and warehouses over the last 20 years, with the new construction covering riverbeds that would have provided natural drainage.

A prosecutor has ordered an investigation into building violations in the area, where two people had already died in flooding that struck in 1996.

"There is a bad precedent with public works in this country," Interior Minister Panos Skourletis told Antenna TV.

Government spokesman Dimitris Tzanakopoulos said "Illegal building was a response to huge social and economic inequality."

Source: AFP