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 as Russian President Vladimir Putin prepares to speak at a major economic event in St Petersburg, and a day after Ukraine's President Zelensky offered face-to-face talks with the Russian leader on ending the war.
Meanwhile, Ukraine has confirmed one of its naval drones exploded off Romania's coast on Friday. No injuries were reported in the blast.
It is unclear if the strikes on cargo ships claimed by Kyiv are linked to the deaths of five Azerbaijanis, whom the country's foreign ministry said were killed in drone attacks on ships in the sea.
Azerbaijan's foreign ministry said in a statement on Friday that two cargo ships, the Natra and Zirkon, were attacked by drones overnight in the Taganrog Bay area of the Sea of Azov.
It did not specify who was behind the attacks and noted that the ships do not belong to Azerbaijan.
Russia has blamed Ukraine for the attacks. There was no immediate response from Kyiv.
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". 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.
Deaths and injuries from Russian strikes were also reported in Kherson, Kharkiv, Sumy, Zaporizhzhia, Chernihiv and Dnipro regions.
Ukrainian President Volodymyr Zelensky said food warehouses, a postal building and a school were among the other facilities that were hit.
On Thursday, he called for a face-to-face meeting between himself and Vladimir Putin in a renewed bid to end the war.
In an open letter to the Russian president, Zelensky said it would be "wrong to simply wait" until the war in Europe becomes the focus of the US's attention once more, adding peace could only come "through direct engagement between" Ukraine and Russia.
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. Spokesman Dmitry Peskov told Russian state newspaper Izvestia that it was likely Putin would comment on it on Friday.
The European Union, France and the US are among those that have backed Zelensky's calls for a meeting.
"I think it would be great if they met. They should. Get it done," US President Donald Trump told reporters on Thursday.
"I want them each to make certain compromises, and I think they're going to do it."
Speaking to foreign journalists in St Petersburg on Thursday, without apparently having seen the contents of the letter, Putin said he was "certainly prepared and willing to reach an agreement with Ukraine", but said compromises needed to be made.
He appeared to immediately cast doubt on whether a meeting or deal could ever take place.
"Whether Mr Zelensky is a legitimate representative of Ukraine, this is a question for the lawyers, for a legal analysis," he said - a repetition of a Russian line that there has been no presidential election since Zelensky's term expired in May 2024.
However, elections have been suspended in Ukraine since martial law was declared after Russia's invasion.
Putin's 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.
Ceasefire negotiations have stalled in recent months, and previous peace talks in Geneva, Abu Dhabi and Istanbul have failed.
Also on Friday, a sea drone exploded in the Romanian Black Sea port of Constanta.
The country's defence ministry said the drone had self-detonated near an oil terminal without causing any casualties, although the 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 was involved, saying it had been knocked off course by Russian electronic interference. There was no immediate response from Moscow to this claim.
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.
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 on the Black Sea coast.
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".
