HELSINKI, Nov 17 (Reuters) - Finland's biggest utility, Fortum, swung to a steeper than expected underlying third-quarter operating loss on Tuesday, burdened by a weak performance at its German subsidiary Uniper.

Fortum said its comparable third-quarter operating result fell from a year ago to a loss of 176 million euros ($209 million), against an average loss of 153 million expected by analysts in a company supplied poll.

"Fortum's operating environment in the third quarter of 2020 was characterised by continued uncertainty caused by COVID-19, low Nordic power prices due to high hydrology, and the seasonally weak third-quarter results of the Uniper segment," Chief Executive Markus Rauramo said in a statement.

Fortum felt the COVID-19 negative impact mainly on its Russian operations, which continued to be adversely effected from lower power and heat demand, while its impact on Nordic power demand was limited, Rauramo said.

"We have also proceeded with the strategic reviews of our Polish, Baltic, and Swedish district heating businesses as planned and are reviewing all alternatives, including possible divestments," he added.

($1=0.8436 euros) (Reporting by Anne Kauranen; Editing by Clarence Fernandez and Louise Heavens)