The sense of dread deepened Tuesday in Ukraine because of warnings that Russia may try to spoil the country’s Independence Day holiday and mark the war’s six-month point with intensified attacks.

The U.S. reinforced the worry with a security alert citing “information that Russia is stepping up efforts to launch strikes against Ukraine’s civilian infrastructure and government facilities in the coming days.” As it has done previously, it urged American citizens to “depart Ukraine now.”

Kyiv authorities banned mass gatherings in the capital through Thursday for fear of missile attacks around Independence Day, which, like the six-month mark in the war, falls on Wednesday. The holiday celebrates Ukraine’s independence from the Soviet Union in 1991.

“Our country is having a very hard time, and we need to be careful,” 26-year-old Vlad Mudrak said in support of the ban.

Anxiety also mounted after the weekend car bombing outside Moscow that killed the daughter of a leading right-wing Russian political theorist. Russia accused Ukraine of carrying out the attack. While Ukraine denied involvement, the bloodshed stirred fears of Russian retaliation.

Hundreds of people paid tribute at a memorial service Tuesday to the bombing victim, Darya Dugina, 29, the daughter of Alexander Dugin, a writer dubbed “Putin’s brain” and “Putin’s Rasputin” because of his purported influence on Russian President Vladimir Putin.

Dugina, a pro-Kremlin TV commentator, died when the SUV she was driving blew up Saturday night as she was returning home from a patriotic festival. Her father, a strong supporter of the invasion of Ukraine, was widely believed to be the intended target.

Over the weekend, Ukrainian President Volodymyr Zelenskyy warned that Russia “may try to do something particularly nasty, something particularly cruel” this week.

On Tuesday, however, Zelenskyy stressed defiance rather than worry when he raised the national flag at a memorial one day ahead of Independence Day.

“The blue and yellow flag of Ukraine will again fly where it rightfully should be — in all temporarily occupied cities and villages of Ukraine,” he said, including the Crimea Peninsula, which Russia annexed in 2014.

He added: “It is necessary to liberate Crimea from occupation. It will end where it had started.”

At a separate event, Zelenskyy appeared to downplay the threats this week, indicating that at most, he expected increased intensity rather than new targets, and he added, “No one wants to die, but no one is afraid of Russia, and this is the most important signal.”

NATO, meanwhile, said Zelenskyy can continue to count on the 30-nation alliance for help in defending itself in what Secretary-General Jens Stoltenberg called “a grinding war of attrition.” The war broke out on Feb. 24.

“This is a battle of wills and a battle of logistics. Therefore, we must sustain our support for Ukraine for the long term so that Ukraine prevails as a sovereign, independent nation,” Stoltenberg said at an international conference on Crimea.

One particular source of foreboding is Europe’s largest nuclear power plant, in southeastern Ukraine, where shelling has raised fears of a catastrophe.

Shelling close to the Zaporizhzhia plant continued early Tuesday. Regional Gov. Valentyn Reznichenko said Russian forces fired on Marhanets and Nikopol, two towns less than a dozen kilometers (7 miles) from the power station. The U.N. Security Council was scheduled to meet Tuesday to discuss the danger.

Another source of concern is the fate of Ukrainian prisoners of war. Michelle Bachelet, U.N. high commissioner for human rights, cited reports that Russia and its separatist allies in eastern Ukraine are planning to put Ukrainian POWs on trial, possibly in the coming days. The Kremlin has denounced Ukrainian prisoners as Nazis, war criminals and terrorists, and several prisoners have been sentenced to death.

In the eastern Ukrainian city of Donetsk, Russian authorities reported four people were killed and nearly a dozen wounded in Ukrainian shelling of a separatist headquarters and other buildings.

A small bright spot emerged in Ukraine: A new soccer season started Tuesday in Kyiv. Shakhtar Donetsk and Metalist 1925 from Kharkiv — teams from eastern cities fighting for their existence — played to a 0-0 draw in a 65,000-capacity downtown stadium with no fans allowed.

“This is work … to show the world that life in Ukraine does not stop but continues,” Shakhtar coach Igor Jovicevic said.