Greece said Thursday that archaeological sites, including the Acropolis, will be closed during the hottest hours of the day due to a new heatwave.

The nation is preparing for further high temperatures until Sunday, with peaks of 43 degrees Celsius (109 degrees Fahrenheit) expected in the centre of the country on Thursday.

"Due to high temperatures, the schedules of archaeological sites... will be modified and (the sites) will close from noon to 5:30 pm (0900-1430 GMT)," Greece's culture ministry said.

In addition, due to a previously announced work stoppage by Acropolis security guards, the site will remain closed until the end of the day from Thursday to Sunday.

The guards' union claimed that at least 20 visitors had fainted at the site due to temperatures that can soar to 45C.

The World Heritage Site is currently experiencing a surge in visitor numbers and had already closed last weekend during the hottest hours.

As Greece announced the restrictions, firefighters were still battling wildfires west of Athens, which have so far burned thousands of hectares.

"We still have a heatwave and strong winds ahead of us, the hard times haven't passed yet, we need to be on alert," warned Prime Minister Kyriakos Mitsotakis