The EC chief said he had raised the issue to the Belarusian delegation at the European Union's (EU) Eastern Partnership summit in Brussels earlier on Friday.
"I have to say as far as the commission is concerned, we do not consider this issue being bilateral issue between Lithuania and neighbouring country, it's a European issue and I am being in full solidarity with Lithuanian president and Lithuanians when it comes to this point," Juncker told a news conference.
"We will follow this very closely, this is not something we would deal with benign and neglect," the EC president added.
As proposed by Lithuania, the demand to respect "the highest international nuclear safety and environmental standards" has been included in the final declaration of the meeting.
Lithuania has been accusing Minsk of building its Astravyets nuclear utility in an unsafe manner close to Lithuania's border and its capital Vilnius. Meanwhile, the Belarusian foreign minister said at the meeting that his country was open to cooperation with the EU on the construction of the power plant.
In an effort to place additional burdens on the Astravyets project, Lithuania has pledged to block access of the unit's electricity to the Lithuanian market. Two years ago, the Lithuanian government turned to Brussels with a request to launch discussions on the EU level about boycotting the electricity from unsafe power plants, however, did not secure support from the commission or other EU countries.
The power plant is being built by Russia's corporation Rosatom.