Ukrainian long-range drones struck an oil terminal in St. Petersburg and set it ablaze, President Volodymyr Zelensky said Wednesday, as the Russian city hosts an annual international economic forum that is a banner event for President Vladimir Putin. The drones flew more than 1,000 kilometres (600 miles) to hit the terminal, Zelensky said on social media. Clouds of black smoke rose over the city’s port after the attack. St. Petersburg is Russia’s second-largest city and is where Putin was born. Read moreLive: Russian warship at Baltic Fleet base struck overnight, Ukraine says Russian authorities said only that the Ukrainian drone strike targeted the city’s infrastructure, without providing further details. The airport of St. Petersburg briefly suspended flights overnight because of the attack. Authorities also cut off mobile internet services. A strike deep in Russian territory With the front line changing little as swarms of drones hinder battlefield movement, both sides have sought an edge by increasingly launching long-range strikes. The war that followed Russia’s invasion of its neighbour has now stretched into its fifth year, with no end in sight. The latest strikes are an embarrassment for Putin, weeks after he pruned back an annual Victory Day parade in Moscow due to fears of Ukrainian drone attacks. Read moreStrikes on Russian strategic sites show Ukraine 'ramping up long-range strikes' Putin is set to speak on Friday at the economic forum in St. Petersburg that the Kremlin views as a prestige event. The gathering has for decades been Russia’s leading event for attracting foreign capital. It is sometimes called Russia’s Davos, likening it to the World Economic Forum held in Davos, Switzerland. Major Western investors and officials have stayed away since Russia invaded Ukraine more than four years ago. Saudi Arabia is a special guest country this year and is due to send a large business delegation. The strikes came a day after Russian forces launched a massive drone and missile attack on Kyiv and other Ukrainian cities, killing at least 22 civilians and wounding 138, as Moscow followed through with its threat of escalating its regular barrages. Kremlin spokesman Dmitry Peskov said Wednesday that Russia’s deep strikes have already taken on a “systematic” character. Increasing frustration over defense weaponry Ukraine is short of American-made Patriot air defense missiles, in part because of US stocks depleted by the Iran war, leaving it vulnerable to Russia's ballistic missiles. Zelensky on Wednesday expressed frustration with his own government's officials, saying there is an agreement “at the highest political level” for the purchase of Patriot systems but implementation is being held up by financial, legal and technical considerations.