Falling energy costs, including from the rise of renewables, are making the controversial expansion of Hungary's sole nuclear plant with Russian reactors less viable, one of the country's energy chiefs said Monday.

"Due to the development of renewable resources, we can see clearly that the price of energy will fall in the future," said the chief executive of Hungary's national electricity company MVM, Peter Csiba, in an interview in the leading Hungarian daily Nepszabadsag.

Construction of the two 1,200 megawatt reactors at the Paks plant outside Budapest is considered a strategic project by Prime Minister Viktor Orban, but is viewed sceptically by the opposition and the European Commission.

The 12.5-billion-euro ($13.9 billion) deal, awarded in 2015 to Russia's Rosatom, was based on a provisional price of electricity of 57 to 65 euros per megawatt/hour, according to Csiba.

Wholesale electricity prices were around 90 to 100 euros per megawatt/hour until quite recently, but "we're around 30 euros now" due to the fall in oil prices and the rise of renewables, said Csiba.

The head of MVM, which operates Paks, indicated he would prefer to invest in renewable energy.

Hungarian opposition groups have critcised the lack of transparency in awarding the contract to Rosatom.

The terms of the deal, due to be financed with a 10-billion-euro loan from Moscow, has been classified by the Hungarian government for 30 years.

Brussels said in November last year it was opening an in-depth probe to determine whether the construction of the reactors was economically justified and ensure there was no illegal state aid.

The Hungarian government said recently it was willing to consider other financing for the reactors, construction of which is due begin in 2018 and be completed by 2023.

Paks, which contains four ageing Soviet-built reactors, currently produces around 40 percent of Hungary's electricity.