The warning, issued on Tuesday, is the first specific security alert issued by the US embassy in Kyiv in recent months. It does not say what places it believes Russia plans to target, but advised US citizens to leave Ukraine if safe to do so.

Ukraine’s President Zelensky has warned Russia could do “something particularly ugly” ahead of Wednesday’s holiday.

Tensions are particularly high following a string of explosions and attacks in Russian-occupied Crimea.

Ukrainian President Zelensky abns celebrations, sounds warning

Events to mark Ukraine’s Independence Day on Wednesday have been banned in the country’s capital, Kyiv, and second-largest city, Kharkiv, as Ukrainian President Volodymyr Zelensky and other officials warn that Russia may carry out missile strikes.

Zelensky said on Saturday that Russia might be planning “particularly ugly” attacks to coincide with the occasion, which will mark 31 years since Ukraine broke its ties with the Soviet Union and as the war nears its six-month milestone.

“We must all be aware that this week Russia could try to do something particularly ugly, something particularly vicious,” Zelensky said in a video message.

“One of the key tasks of the enemy is to humiliate us, Ukrainians, to devalue our capabilities, our heroes, to spread despair, fear, to spread conflicts … Therefore, it is important never, for a single moment, to give in to this enemy pressure, not to wind oneself up, not to show weakness,” he said.

Kyiv is far from the front lines and has only rarely been hit by Russian missiles since Ukrainian troops repelled a Russian ground offensive to seize the capital in March.

Other jurisdictions also restricted public gatherings. In Kharkiv, a northeastern city that has come under frequent and deadly longer-range artillery and rocket fire, Mayor Ihor Terekhov announced an extension to an overnight curfew to run from 4pm to 7am, effective from Tuesday to Thursday.

In the port of Mykolaiv, near Russian-held territory to the south, regional governor Vitaliy Kim said authorities planned a precautionary order for residents to work from home on Tuesday and Wednesday and urged people not to gather in large groups.

Car bomb in Moscow blamed on Ukraine heightens tensions

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.

Russian politicians bade farewell at a service on Tuesday to Darya Dugina, the slain daughter of one of Russia’s most prominent nationalist ideologues, hailing her as a martyr whose death must inspire Russian forces fighting in Ukraine.

Russian demands for retribution came as Ukraine prepared to mark 31 years of independence on Wednesday, which coincides with six months since Russia’s invasion of Ukraine began.