Ukraine has alleged that Russian drones landed on Romanian territory during a series of strikes on a neighbouring Ukrainian city.
Foreign Minister Dmytro Kuleba told reporters after a news conference in Kyiv that Ukraine had photographic evidence supporting its claim.
However, Romania has rejected Kyiv's version of events and BBC Verify says it cannot authenticate the image.
The row came as the Russian and Turkish leaders held talks.
Turkish President Recep Tayyip Erdogan had travelled to the Russian city of Sochi in an attempt to persuade President Vladimir Putin to revive the deal that allowed Ukraine to export grain safely through the Black Sea.
Mr Putin said the deal, which Moscow abandoned in July, would not be reinstated until the West met his demands for sanctions to be lifted on Russian agricultural produce.
But he did say that Russia was moving ahead with plans to supply free grain to six African countries "and even carry out logistics free of charge".
The stymied grain deal is also at the heart of the dispute between Ukraine and Romania over crashed drones.
Russia has been hitting Ukraine's port facilities along the River Danube for more than a month, trying to prevent Ukraine from using the river to export its grain.
With most ships prevented from using Ukraine's Black Sea ports, Moscow seems intent on preventing Kyiv from developing viable alternative routes.
Sunday night's attacks on the port of Izmail were launched just a day after Russian drones hit the nearby port of Reni.
Since the latest attack, something of a war of words has followed between Ukraine - which insists that one or more drones landed across the river, inside Romania - and the government in Bucharest, which says it didn't happen.
"Of course, there is a risk, because what happened there is very close to our borders," said Romanian Foreign Minister Luminita Odobescu.
"We have seen that Russia cynically continues to attack the civilian infrastructure, not allowing Ukraine to export their cereals.
"Of course, there is a risk of accidents or incidents, but for the time being, it was not the case."
Mr Kuleba, sounding angry, said it was "absolutely obvious" what had happened.
He suggested that some of Ukraine's partners were, in effect, turning a blind eye in order to avoid being involved in the conflict.
If a Russian drone did land on Romanian territory, without it being the result of an interception, then this would mark the first time Russia has directly, if accidentally, hit a Nato member state.
Last November a missile landed in Poland during a Russian air attack. Ukraine initially claimed it was a Russian missile but it was later found that this was likely to have been a Ukrainian air defence missile.
BBC Verify has been examining a still image and a video purporting to show the incident. The image was published to social media on Monday morning by Oleg Nikolenko, a spokesperson for Ukraine's Ministry of Foreign Affairs.
Both the image and the video show a fireball rising over a forested riverbank at night, but the visual evidence is of very low quality, in part because it was shot in the dark, from a distance, and in part because it is low-resolution. Additionally, the video appears to have been blurred in places, obscuring certain details.
As a result, BBC Verify cannot confirm Ukraine's claims, nor whether the explosion was caused by a drone or by something else.