Finnish officials have confirmed that a Russian warship was attacked in the Gulf of Finland on Wednesday, although little is known about the strike.
Ukraine's SBU security service said it targeted the ship Naro-Fominsk as part of a broader drone operation against Russia's Ust-Luga oil export depot on the Baltic coast.
The attack, thought to be the first time Ukrainian drones have attacked a Russian ship in an area of the Baltic Sea open to civilian maritime traffic, has raised questions about whether Ukraine could expand its strikes.
Helsinki has been one of Kyiv's closest European allies, and Finnish officials have described the incident as a direct result of Russia's continued full-scale war.
Finland's military has given few details of the incident, but it appears to have taken place in a narrow stretch some distance from the Finnish coast.
The country's Foreign Minister Elina Valtonen was adamant that the situation in the Baltic Sea remained stable and safe and that there was no threat of any kind to marine traffic.
Defence officials are understood to have stressed to Kyiv that the Gulf is a relatively narrow waterway used by oil tankers and civilian vessels, and is also home to subsea data cables.
Ukraine's SBU described the Russian ship as a Buyan-M type small corvette that carried Kalibr and Oniks cruise missiles. There were also unconfirmed reports of Russian combat helicopters being destroyed.
Martin Herem, the defence minister of Estonia, which borders the Gulf, told public broadcaster ERR that the ship was hit at about 05:30 (03:30 GMT) on Wednesday but did not say how it was attacked: "We know that smoke came from the ship afterwards. The ship itself went back towards St Petersburg."
The extent of casualties or damage sustained by the Russian navy ship remains unclear, and Moscow rarely gives such details.
"The Baltic Sea fleet has been untouched up to this point but there seemingly was a window of opportunity for Ukraine and they understandably tried to use it," said military analyst Emil Kastehelmi, co-founder of the Black Bird monitoring group.
Russian and Ukrainian vessels have been repeatedly targeted in the Black Sea since Russia's full-scale invasion began in February 2022, but even though the latest strike on a ship in the Baltic is unprecedented, Kastehelmi told the BBC he did not consider it a serious escalation.
Ukrainian drones are regularly launched into Russia from northern Ukraine. Several storage tanks at the Ust-Luga terminal are thought to have been hit overnight on Wednesday and fires at the port were visible over the border in Estonia.
Herem said Ukrainian drones attacked the Ust-Luga region from midnight until about 06:00, mostly flying close to the Estonian border but still in Russian airspace.
He added that one drone had entered Estonian airspace briefly from Latvia and then headed towards Russia.
Nato aircraft based in the Baltic states were in the air at the time, but Herem said it had not been considered safe for them to destroy the drones.
Ukrainian foreign ministry spokesman Heorhiy Tykhyy said late on Wednesday that Moscow's allegations that Kyiv had used the airspace of the Baltic states to attack Russia were "baseless", although he acknowledged "incidents might occur when Russia diverts drones from its airspace using electronic warfare".
