ST. PETERSBURG, Russia — A massive black cloud rising above the St. Petersburg skyline from a Ukrainian drone strike set a gloomy tone for the opening of President Vladimir Putin’s annual showcase of Russia’s economic achievements.

With Putin set to arrive Thursday in his hometown that is hosting the St. Petersburg International Economic Forum, the Ukrainian attack a day earlier that set an oil terminal ablaze was another embarrassing blow to his efforts to minimize the impact of the 4-year-old conflict and cast it as a distant event with no effect on Russian daily life.

The attack, which also targeted a naval base near Russia’s second-largest city on the Gulf of Finland, underlined Ukraine’s growing capability to hit deep inside its neighbor and demonstrated that even the heavily protected city where Putin was born is increasingly vulnerable.

Scores of flights were delayed or diverted at St. Petersburg’s airport and authorities cut cellphone internet service to try to prevent drone attacks.

Putin had scaled down Russia’s annual Victory Day parade on May 9, fearing Ukrainian drone strikes. Days later, a massive drone attack on Moscow’s suburbs killed three and showed the capital’s vulnerability.

Kremlin spokesman Dmitry Peskov said Russia’s forces were pressing inside Ukraine “in order to prevent such attacks” like the one on St. Petersburg. He noted that “systematic” strikes on Kyiv that Russia threatened last week are underway.

On Tuesday, Russia hit Kyiv and other Ukrainian cities with hundreds of drones and dozens of missiles, killing 23 and wounding 151 others.

Putin has used the forum to showcase his country’s economic advances and encourage foreign investment. Often styled as the Russian version of the World Economic Forum in Davos, Switzerland, it usually draws tens of thousands of delegates from around the world.

While Western officials and business people have stayed away from the forum after Putin sent troops into Ukraine in 2022, Russia has sought to attract more guests from other regions to underline its declared goal of promoting a “multipolar world.”

Saudi Arabia, which is a special guest this year, has sent a large delegation. The presidents of Uzbekistan and Tanzania and China’s vice president also are attending. A U.S. official, Rodney Mims Cook Jr., head of the U.S. Commission of Fine Arts, will attend the event for the first time in years.

Russia’s economic outlook has clouded as the initial boost from massive military spending has fizzled. The government has raised taxes and increased domestic borrowing to keep its budget deficit under control.

Putin is expected to minimize Russia’s economic problems during his forum appearance, but the Ukrainian attack on St. Petersburg’s port about 15 kilometers (about 9 miles) from the forum’s venue have underlined the growing challenges posed by the conflict.

Hours before the forum opened Wednesday, Ukrainian drones also hit the Kronstadt naval base on an island in the Gulf of Finland, the home of Russia’s Baltic Fleet since Peter the Great founded St. Petersburg. While most of the fleet has moved to Russia’s Baltic exclave of Kaliningrad, Kronstadt retains its symbolic importance as the seat of the country’s naval glory, with its historic cathedral and old fortifications.