Serbian President Aleksandar Vucic accused Ukraine and an unidentified EU country of being behind a series of hoax bomb threats against Air Serbia planes.
Since Russia launched its invasion of Ukraine in late February, more than a dozen Air Serbia flights have been forced to return to Belgrade or Moscow due to bomb threats, and Belgrade's airport was evacuated at least three times.
"Foreign (intelligence) services of two countries are doing that. One is an EU country, and Ukraine is another one," Vucic said late on Sunday, without providing evidence.
In a statement, Oleg Nikolenko, a spokesman for Ukraine's foreign ministry, dismissed Vucic's remarks as "baseless" and "untrue."
He also said Ukraine was disappointed with Serbia's refusal to join EU sanctions against Russia.
"We call on Belgrade to stand up for the truth and fully join in support of Ukraine and in upholding the values on which united democratic Europe is founded," it said.
Serbia, a candidate for EU membership, is almost entirely dependent on Russian gas and oil. It refuses to impose sanctions against the Kremlin and it maintains regular flights to Moscow.
"We are continuing these flights literally out of principle, because we want to show that we are a free country and we make our own decisions," Vucic said.
"Do not decide for us when to cancel flights."
In recent weeks, Belgrade has voted three times for UN resolutions that condemned Russia's invasion of Ukraine and suspended it from the main UN human rights body.
Moscow calls its actions in Ukraine a special military operation to denazify the country.