Ukraine on Monday rejected as baseless and false the accusations made by Serbia’s president that Ukraine’s secret service is behind a series of hoax bomb threats against Air Serbia flights to Russia.

Serbian President Aleksandar Vucic has claimed that the foreign intelligence services of Ukraine and an unidentified European Union nation “are doing that. The pro-Russian Serbian leader did not provide evidence for his claim. Other Serbian officials had alleged that the threatening bomb hoax emails have been sent to Serbia from either Ukraine or Poland.

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“His (Vucic’s) statements about Ukraine’s alleged involvement in bomb threats to Serbian air carriers flying to Russia are false,” Ukraine’s Foreign Ministry spokesman Oleg Nikolenko said in a statement.

The Serbian national carrier is the only European airline besides Turkish air companies that has not joined EU flight sanctions against Russia over its war in Ukraine.

Several Air Serbia flights to Moscow and St. Petersburg have been delayed or had to return to Belgrade after the anonymous bomb threats.

Vucic said although the flights to Russia are not making a profit because of frequent returns to their base in the Serbian capital, the flights will continue “as a matter of our principle.”

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Serbia has voted in favor of three U.N. resolutions condemning Russia’s bloody carnage in Ukraine, but has so far rejected joining international sanctions against its allies in Moscow.

Ukraine’s foreign ministry spokesman expressed disappointment that Serbia, a candidate to join the EU, has not yet supported the 27-nation bloc’s sanctions imposed on Russia.

“Tough sanctions and unity of the democratic world can stop this war,” Nikolenko said in a statement. “We call on Belgrade to stand up for the truth and fully join in supporting Ukraine and upholding the values on which a united democratic Europe has been founded.”