On the eve of Ukraine's independence day and the half-year mark of Russia's invasion of its neighbor, there was increasing unease in the country on Tuesday that Moscow could be centering on specific government and civilian targets during the holiday.

The United States reinforced those concerns when its embassy in Kyiv issued a security alert, saying it “has information that Russia is stepping up efforts to launch strikes against Ukraine’s civilian infrastructure and government facilities in the coming days.”

Over the weekend, Ukrainian President Volodymyr Zelenskyy already sensed a threat coming when he said in his daily address that “we should be aware that this week Russia may try to do something particularly nasty, something particularly cruel.”

The warnings come on the heels of Russia's claim that Ukrainian intelligence was responsible for the car bombing that killed the daughter of a leading right-wing Russian political thinker over the weekend. Ukraine denied involvement.

Darya Dugina, a 29-year-old commentator with a nationalist Russian TV channel, died when a remotely controlled explosive device planted in her SUV blew up on Saturday night as she was driving on the outskirts of Moscow.

The sense of dread pervading the war centers in part on Europe’s largest nuclear power plant, at Zaporizhzhia in southeastern Ukraine, where continued shelling and fighting in the area has raised fears of a nuclear catastrophe.

Read:Six months on, Ukraine fights war, faces painful aftermath

U.N. Secretary General Antonio Guterres late Monday warned about the nuclear threat in general, particularly since Russia alluded to its massive nuclear arsenal early in the war.

Guterres demanded a halt to “nuclear saber-rattling” on Monday, saying the world is at a “maximum moment of danger” and all countries with nuclear weapons must make a commitment to “no first-use."

That didn't prevent shelling close to Zaporizhzhia early Tuesday. Regional governor Valentyn Reznichenko said Russian forces fired on nearby Marhanets and Nikopol on the right bank of the Dnieper River, continuing weeks of relentless overnight shelling.

Amid the death and destruction, there was one small point of light. All professional soccer was stopped in February, but a new league season starts Tuesday in Kyiv.

The Olympic Stadium will see the the opening-day meeting of Shakhtar Donetsk and Metalist 1925 from Kharkiv — teams from eastern cities that are fighting for their very existence.

No fans will be allowed in the 65,000-capacity downtown stadium for the kickoff at 1 p.m. local time, and the players must be rushed to bomb shelters if air-raid sirens sound.

"The teams, the players will be proud of this event,” Shakhtar captain Taras Stepanenko said Monday in a telephone interview with The Associated Press.