Russia and Ukraine traded drone attacks early Wednesday, officials said, with Kyiv apparently targeting Moscow again and the Kremlin's forces launching another bombardment of Ukrainian grain storage depots in what have recently become signature tactics in the almost 18-month war.

Later Wednesday, the Ukrainian intelligence agency claimed it had destroyed a key Russian S-400 surface-to-air missile defence system in occupied Crimea. If confirmed, it would be another embarrassing blow for Moscow, as Ukraine increasingly targets Russia's assets far behind the front line in southern and eastern Ukraine.

The agency, known by its acronym GUR, claimed on its official Telegram channel that Russia has a “limited number” of the sophisticated systems and that the loss “is a painful blow.” Moscow officials made no immediate comment.

The long-range S-400 missiles are capable of striking enemy aircraft and are regarded as one of the best such systems available. They have a range of 400km and can simultaneously engage multiple targets.

Earlier, a three-hour nighttime Russian drone attack in Ukraine’s southern Odesa region overnight Tuesday caused a blaze at grain facilities, Odesa Regional Military Administration Head Oleh Kiper said.

The attack destroyed 13,000 metric tons of grain, bringing the month’s total grain losses to around 270,000 metric tons, Ukrainian Infrastructure Minister Oleksandr Kubrakov said in a Facebook post.

Russia zeroed in on Odesa last month, crippling significant parts of the port city's grain facilities, days after President Vladimir Putin broke off Russia's participation in the Black Sea Grain Initiative. That wartime deal enabled Ukraine’s exports to reach many countries facing the threat of hunger.

Under a year of that deal, Ukraine shipped 32.9 million metric tons (36.2 million U.S. tons) of grain, most of it from the Odesa region.

Russian officials, meanwhile, claimed to have downed Ukrainian drones in Moscow and the surrounding region early Wednesday, the defence ministry and the mayor said. No casualties were reported in the drone attack, which has become almost a daily occurrence in the Russian capital.

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Moscow Mayor Sergey Sobyanin said one drone smashed into a building under construction in Moscow City, a prestigious business complex hit by drones twice before. Several windows were broken in two buildings nearby and emergency services responded to the scene.

Russia’s Ministry of Defence said the drone had been electronically jammed.

It blamed the attack on Ukraine and said two other drones were shot down by air defence systems in the Mozhaisk and Khimki areas of the Moscow region. Kyiv officials, as usual, neither confirmed nor denied Ukraine was behind the drone attacks.

Moscow airports briefly closed but have now reopened, according to Russian state media.

Neither side’s claims could be independently verified.

Ukraine has since early this year sought to take the war into the heart of Russia. It has increasingly targeted Moscow’s military assets behind the front lines in eastern and southern Ukraine and at the same time has launched drones against Moscow.

Meanwhile, a Russian drone attack on the city of Romny in northeastern Ukraine struck a local school, killing the principal, his deputy, a secretary and the school librarian, according to Ukraine’s Ministry of Internal Affairs.

Also, three people were killed in the Belgorod region of Russia on the Ukrainian border after repeated shelling of a sanatorium, according to Governor Vyacheslav Gladkov.

Gladkov said the sanatorium in the village of Lavy, about 40km from the border, was shelled, killing two refugees and a staff member.

The Belgorod region has witnessed sporadic fighting and shelling during the war, including a border incursion last May that prompted the Kremlin to introduce tighter security.

A handful of foreign dignitaries, including the prime minister of Finland and the presidents of Portugal and Lithuania, visited Ukraine on Wednesday.

Their presence coincided with the Day of the National Flag of Ukraine, which precedes Ukrainian Independence Day on Thursday.

Ukrainian President Volodymyr Zelenskyy, along with Ukrainian Armed Forces Commander in Chief Valerii Zaluzhnyi and other top officials, attended the unfurling in Kyiv of a giant Ukrainian flag with numerous signatures of soldiers, volunteers, doctors and rescuers.