Russia will regard Western F-16 fighter jets sent to Ukraine as a "nuclear" threat because of their capacity to carry atomic weapons, Foreign Minister Sergei Lavrov said on Thursday.
Modern warplanes have been at the forefront of Kyiv's demands for military aid from its Western allies as it fights back against Russia's offensive.
Lavrov spoke of a US plan to transfer F-16s to Ukraine, although Washington has not given the go-ahead for any country to supply them.
"Russia cannot ignore the ability of these aircraft to carry nuclear weapons. No amount of assurances will help here," Lavrov was quoted as saying by the Russian foreign ministry.
"In the course of combat operations, our servicemen are not going to sort out whether each particular aircraft of this type is equipped to deliver nuclear weapons or not.
"We will regard the very fact that the Ukrainian armed forces have such systems as a threat from the West in the nuclear sphere."
The Netherlands and Denmark are leading a plan to train Ukrainian pilots on using the US-made aircraft as part of an 11-nation coalition.
The programme will begin in Denmark in August after the United States authorised the move.
