Ukraine has started constructing two US-designed reactors at a nuclear power plant in the west of the country following devastating Russian attacks on its energy network.

Workers at Khmelnytskyi nuclear power station on Thursday poured a symbolic cubic metre of concrete for the reactors, which will use US technology and fuel, as Ukrainian and US flags fluttered.

The project is aimed at increasing Ukraine's energy security and reducing its dependence on Russian nuclear technology.

Guests at the ceremony included the head of Ukraine's state nuclear operator, Energoatom, Petro Kotin; Ukrainian Energy Minister German Galushchenko; US Ambassador to Ukraine Bridget Brink and the CEO of US nuclear energy company Westinghouse, Patrick Fragman.

Energoatom asked for the launch to be reported Saturday due to security concerns over Russian attacks.

The ceremony took place hours after a Russian strike destroyed a power plant outside Kyiv.

"This is our answer to the terrorists, this is our answer to the invaders," Galushchenko said.

"Today we are talking about the future, we are talking about stability, we are talking about the development of nuclear energy," he said, describing the plans for Khmelnytskyi as Ukraine's most significant modernisation project since World War II.

A sign at the plant reads "the future is with nuclear energy".

Ukraine derives around half of its energy from nuclear power. All its current reactors are of Soviet design but it has shifted them to US fuel.

The country's largest nuclear plant -- Zaporizhzhia in the south -- has been under Russian control since the start of the war.

Khmelnytskyi power plant already has two functioning reactors. Its reactors 3 and 4 are partially built to a Soviet design.

The ground-breaking ceremony was for unbuilt reactors 5 and 6, which will be built using Westinghouse's AP1000 technology.

The units will each have a capacity of over 1,100 megawatts and according to Fragman will be "the most advanced nuclear reactor operating today in the world".

With all six reactors on stream, Khmelnytskyi will become Europe's largest plant, topping Zaporizhzhia, said Energoatom chief Petro Kotin.

EnergoAtom and Westinghouse did not say exactly when the reactors will come on line.

Energoatom said construction of one reactor unit in peace time takes five years and the estimated cost starts from $ five billion.

The first unit will be on an "accelerated schedule", because Ukraine has agreed to buy heavy equipment that is ready to instal, Fragman said.

The US Exim Bank will partly finance the project.

The Khmelnytska region has been regularly targeted by Russian attacks because of a major air base there.