Ukraine said on Monday Russian drones detonated on the territory of Nato member Romania during an overnight air strike on Ukrainian port infrastructure on the Danube River, but Bucharest denied its territory was hit.
Reuters could not independently verify either account.
Moscow has conducted long-range air strikes on targets in Ukraine since the start of its invasion last year. Since July, when Moscow abandoned a deal that lifted a de facto Russian blockade of Ukraine's Black Sea ports, it has repeatedly struck Ukrainian river ports that lie across the Danube from Romania.
Monday's Russian air strike came hours before president Vladimir Putin was due to discuss reviving the Black Sea deal with the deal's sponsor, Turkish president Tayyip Erdogan.
"According to Ukraine's state border guard service, last night, during a massive Russian attack near the port of Izmail, Russian 'Shakheds' fell and detonated on the territory of Romania," foreign ministry spokesperson Oleg Nikolenko said, referring to Iranian-made drones.
"This is yet another confirmation that Russia's missile terror poses a huge threat not only to Ukraine's security, but also to the security of neighbouring countries, including NATO member states," he wrote on Facebook.
Mr Nikolenko published a photo showing flames of an explosion visible from across a river. Reuters could not immediately verify the vantage point of the image.
The Romanian defence ministry said Romania was not hit.
"The ministry of defence categorically denies information from the public space regarding a so-called overnight situation during which Russian drones would have fallen in Romania's national territory.
"At no time did Russia's means of attack generate direct military threats on Romanian national territory or waters."
Nato has a collective defence commitment under which the military alliance considers an attack on one ally to be an attack on all allies.
Ukrainian lawmaker Oksana Savchuk told Ukrainian television that she believed the Romanian denial could be part of an effort by Nato to prevent a slide into a direct war with Russia.
Damaged warehouses, buildings ablaze
Kyiv officials said the attack damaged Ukrainian warehouses and set buildings on fire, hours before Erdogan was due to meet Putin in Russia's Black Sea resort of Sochi. Turkey, also a member of Nato, has sponsored the Black Sea grain export deal, and Erdogan has said he expects to persuade Putin to rejoin it.
Russia quit the deal in July that had allowed Ukraine to safely export food across the Black Sea during the war. The Danube has since become a vital export corridor for Ukrainian grain and Russia has targeted the route with regular air strikes.
In the early hours of Monday, the Ukrainian border guard service said two drones hit Romanian soil near Ukraine's Izmail port, the Interfax Ukraine news agency reported.
It said it had relayed the information to its Romanian counterparts, but received no reply.
The Ukrainian president's chief of staff Andriy Yermak said the incident showed the need to increase supplies of modern air defence and long-range weapons to deprive Russia of the ability to launch drones and missiles as Ukraine.
"Additional weapons and long-range missiles for Ukraine - to speed up the de-occupation of our territories. Russia must be defeated on the battlefield," Mr Yermak wrote on the Telegram messaging app.
