Ukraine says it has struck five ships carrying illegal cargo in the Sea of Azov and in coastal waters of Russian-occupied territories.
The Ukrainian drone forces commander said the vessels were involved in "stealing" Ukrainian grain, as well as transferring military cargo and fuel.
The strikes come a day after Ukraine's President Volodymyr Zelensky offered face-to-face talks with Vladimir Putin on ending the war, an offer the Russian leader rejected while visiting St Petersburg for an economic forum.
Meanwhile, Ukraine has confirmed one of its naval drones exploded off Romania's coast on Friday. No injuries were reported in the blast.
Zelensky wrote alongside his latest offer of peace talks that Russians had become tired of Ukrainian drone and missile attacks, petrol shortages and rising prices that the war, now in its fourth year, had brought.
Ukraine has repeatedly targeted military and energy infrastructure within Russia in a bid to limit its war-fighting capability.
Putin's appearance in St Petersburg comes a day after Kyiv had launched a drone attack on the outskirts of the city.
Azerbaijan's foreign ministry confirmed five of its citizens had been killed in attacks on two of the vessels in the Sea of Azov.
It did not specify who had been behind the attacks and noted that the ships did not belong to Azerbaijan.
Earlier, Robert Brovdi, Ukraine's drone commander, announced that five "illegally loitering vessels" had been struck overnight in the ports of Mariupol, Berdyansk and in the coastal waters of what Ukraine calls the "temporarily occupied territories" - parts of the country that Russia currently controls.
He said the names of the five ships, which included cargo vessels and tankers, were painted over and their radars were turned off "with the aim of quietly stealing Ukrainian grain", as well as "transferring military cargo and fuel". The Azeri foreign ministry named two of the ships as the Nastra and the Circon.
Brovdi did not mention any deaths.
In Ukraine, at least 13 people have been killed and more than 70 others have been injured in the past day, according to local officials.
Among them are four people who died after Russian drones struck a dairy factory outside the Ukrainian capital, Kyiv, according to the regional head, and a 35-year-old woman in a drone attack on a petrol station in Kherson, local officials said.
Meanwhile, a naval drone exploded in the Romanian Black Sea port of Constanta on Friday.
The country's defence ministry said the drone had self-detonated near an oil terminal without causing any casualties, although authorities have said it caused considerable damage to a ship and warehouses.
Adrian Teodor Picoiu, Constanta's top official, told G4Media that "information from the Ukrainian side" was that the drone was part of a group of five, with a second one exploding in Ukraine.
Ukraine later confirmed one of its naval drones had been involved, saying it had been knocked off course by Russian electronic interference. Moscow has yet to comment.
The remaining three drones were unaccounted for but officials said there was no further risk. No reason has yet been offered for why the drones would be in Romanian waters.
Romanian President Nicusor Dan wrote on X that it was the second "significant security incident this week", after a stray mine was discovered on a beach near the village of Vama Veche, more than 50km (31 miles) north of Constanta.
It also comes a week after two people were injured when a drone hit a Romanian apartment block in the eastern city of Galati - close to the border with Ukraine.
Romanian officials said they had confirmed it was a Russian drone but Moscow said "accusations" of its involvement were "unsubstantiated".
Zelensky told Putin in an open letter that it would be "wrong to simply wait" until the war in Europe became the focus of US attention once more, adding peace could only come "through direct engagement".
He also called for a full ceasefire for the duration of proposed negotiations - something Putin ruled out earlier on Thursday.
The Kremlin confirmed it had received the letter. Putin said on Friday that he currently saw no reason to meet Zelensky.
The Russian president has previously said compromises would be needed for peace to be achieved.
His longstanding position has been that Ukraine should withdraw from four regions largely occupied by Russia - Donetsk, Luhansk, Kherson and Zaporizhzhia - and give up its efforts to join Nato.
Ukraine has ruled out ceding territory, saying it would embolden Russia to invade again, as it had in 2022 when it launched its full-scale war eight years after illegally annexing Crimea.
The EU, France and the US are among those that have backed Zelensky's calls for a meeting.
