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

The U.S. will announce its largest single aid package for Ukraine on Wednesday, according to two U.S. officials. The package is expected to be valued at roughly $3 billion -- though one official told ABC News some changes could be made overnight, and $3 billion is on the higher end of the estimates.

A senior U.S. official told ABC News the package will come from Ukraine Security Assistance Initiative funds. Unlike presidential drawdown packages, which pull from existing U.S. equipment stocks, the USAI money can be spent on contracts with the defense industry to produce new equipment for Ukraine.

The U.S. has committed about $10.6 billion in military aid to Ukraine since the beginning of the Biden administration.

-ABC News' Matt Seyler and Shannon Crawford

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.