Russia-Ukraine updates: Russian missiles hit close to nuclear reactors: IAEA director

The United States is once again urging its citizens to leave Ukraine amid concerns Russia is ramping up attacks on civilians in the war-torn country.

In a security alert posted Tuesday on its website, the U.S. Embassy in Kyiv urged Americans "to depart Ukraine now using privately available ground transportation options if it is safe to do so."

"The Department of State has information that Russia is stepping up efforts to launch strikes against Ukraine's civilian infrastructure and government facilities in the coming days," the embassy said in the alert. "Russian strikes in Ukraine pose a continued threat to civilians and civilian infrastructure."

Sources within the State Department said the heightened risk of a Russian strike on highly populated centers is most directly tied to Ukraine's Independence Day on Wednesday.

"The risks are really high," Ukrainian President Volodymyr Zelenskyy told ABC News. "We are receiving information that there may be a provocation by the Russian Federation, by the occupiers. Therefore, we do not want large gatherings on such days. The days are beautiful, but ... our neighbors are not."

Russia’s FSB is accusing Ukrainian special services of assassinating Darya Dugina, the daughter of Putin ally Alexander Dugina, who was killed by an explosive this weekend.

The FSB said a Ukrainian national arrived in Russia on July 23 with her 12-year-old daughter and rented an apartment in the same Moscow building where Dugina lived, Russia's state-run RIA Novosti reported. The woman allegedly trailed Dugina for nearly a month and then immediately left for Estonia with her daughter just after this weekend’s bombing.

Russian President Vladimir Putin said in a statement that Darya Dugina was "a bright, talented person with a real Russian heart - kind, loving, sympathetic and open."

Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, said Ukraine is working under the assumption that Russian secret services are behind the killing, saying "Ru-propaganda lives in a fictional world."

-ABC News' Anastasia Bagaeva and Oleksii Shemyskyo

Air raid sirens are sounding across Ukraine.

Ukrainian President Volodymyr Zelenskyy warned Russia could launch a "particularly ugly" provocation this week as Ukraine approaches its Independence Day on Wednesday.

In Kyiv, all public events are canceled and government employees have been told to work from home through the week.

In Kramatorsk, public events have been canceled for Tuesday through Thursday and public transportation has been stopped.

An explosive device planted on the underside of Putin ally Alexander Dugin's vehicle was remotely triggered, Russian investigators said.

Dugin's daughter, Daria Dugina, was killed in a blast near Moscow on Saturday.

"A presumed explosive device planted on a Toyota Land Cruiser went off when the car was moving at full speed past Bolshiye Vyazemy in the Odintsovo urban district at about 9 p.m. on August 20, and the car caught fire," the Russian Investigative Committee said in a statement posted to Telegram. "The woman driving the car died instantly. The victim was identified as journalist, political analyst Daria Dugina."

Alexander and Daria attended a traditional patriotic festival on Saturday afternoon, according to the Odinstovo administration. They'd planned to leave together in the same vehicle, but Daria instead drove alone.

The Russian Investigative Committee's press service told Interfax that Daria was assassinated.

Detectives established that the bomb was planted on the underside of the driver's side of the vehicle, the committee said. Russian media outlets had reported that the SUV belonged to Dugin.

"Detectives and specialists from the Main Forensic Department of the Russian Investigative Committee are continuing to examine the incident scene. In particular, a forensic technician examined the charred vehicle before it was taken to a special parking lot," the Committee said.

Biological, genetic, physical, chemical and explosive examinations have been scheduled, the committee said.

-ABC News' Anastasia Bagaeva