At least three people were killed and seven others injured in a Ukrainian attack on Russian-controlled Crimea, according to Moscow-backed local authorities on Thursday. The strike occurred in Simferopol, where several non-residential buildings were hit. Emergency services were deployed to the site, said Sergei Aksyonov, the Moscow-appointed head of Crimea’s administration.

The attack came a day after Ukraine targeted fuel and military facilities in Russia’s St. Petersburg, where the annual St. Petersburg International Economic Forum (SPIEF) was underway. The event was expected to host around 20,000 delegates from 130 countries, with Russian President Vladimir Putin scheduled to deliver a keynote speech on Friday.

Kremlin spokesperson Dmitry Peskov warned of a “systematic” response to Ukraine’s strikes. Black smoke was seen over St. Petersburg following Wednesday’s attack, visible even from the forum venue.