The Kremlin has threatened to destroy any fighter jets given to Ukraine by its allies, after two countries promised Soviet-era planes, the media reported.
Slovakia became the second Nato country to pledge Kyiv some MiG-29 aircraft on Friday, a day after Poland. Slovakia's fleet was grounded last year and it no longer uses the jets, the BBC reported.
Ukraine has asked Western countries for modern jets like the F-16, but because of long training times, these are seen only as long-term options.
It sees the extra aircraft as key to its defences and possible counter-attacks, a year into Russia's invasion, the BBC reported.
Other Nato countries are considering sending Soviet-era planes such as the MiG, which Ukrainian pilots are trained to fly.
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But Kremlin spokesman Dmitry Peskov condemned the plans, saying that the aircraft would not affect the outcome of Moscow's "special military operation", as it calls the war, but only "bring additional woes for Ukraine and the Ukrainian people".
"Of course, during the special military operation, all this equipment will be subject to destruction," he added, the BBC reported.
At the start of Russia's full-scale invasion, Ukraine was believed to have about 120 combat capable aircraft - mainly ageing MiG-29s and Su-27s.
On Friday, Slovakian Prime Minister Eduard Heger tweeted that his government had approved sending the country's 13 MiG-29s.
He said "promises must be kept" and that he was glad others were answering Ukrainian President Volodymyr Zelensky's call for more weapons.
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He said Slovakia would also send Ukraine part of its Kub air defence system, the BBC reported.
On Thursday, Poland pledged four MiG-29s, to be sent in the coming days, but more are expected to follow.
It is not clear how many of the Slovakian planes are currently operational.