Russian forces have continued to attack energy facilities in Ukraine, causing power outages in seven regions.

The deputy head of Ukraine's presidential office, Kyrylo Tymoshenko, said on social media on Saturday that about 1.5 million households had lost power in the seven regions, including Khmelnytskyi in the country's west and Mykolaiv in the south.

Ukraine's energy minister said on Friday that Russian forces had struck 30 to 40 percent of his country's power infrastructure. He also said that Ukraine had lost at least half of its thermal power generation capacity.

President Volodymyr Zelenskyy said on Saturday that the main target of what he called "terrorists" is energy, and reiterated his call for people to save power.

A US think tank, the Institute for the Study of War, said in a report released on Saturday that the ongoing Russian strikes on Ukrainian infrastructure are "extraordinarily unlikely to erode the Ukrainian will to fight."

But the document noted that those strikes "will increasingly pose an economic and humanitarian challenge for Ukraine as temperatures drop."

It has also been revealed that Russia is building an extensive fortified line of defenses in the eastern region of Luhansk, which the Kremlin has unilaterally annexed.

Britain's defense ministry said in its intelligence update on Sunday that the owner of a Russian private military company, Wagner Group, claimed that his engineering team was constructing the line of defenses. The company is said to have ties with Russian President Vladimir Putin's administration.

The British defense ministry said imagery showed a section of newly constructed anti-tank defenses and trench systems.

It added that "the project suggests Russia is making a significant effort" to prepare defenses behind the current frontline, "likely to deter any rapid Ukrainian counteroffensives."