BELGRADE (REUTERS) - Serbia's president Aleksandar Vucic accused Ukraine and an unidentified European Union country late on Sunday (April 17) of being behind a series of false bomb threats to Air Serbia passenger planes.

Since Russia launched its invasion of Ukraine in late February, more than a dozen Air Serbia flights were 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 the another one," Mr Vucic said without providing evidence.

Serbia, which is almost entirely dependent on Russian gas and oil, refuses to impose sanctions against the Kremlin and it maintains regular flights to Moscow.

Mr Vucic said it will continue to do so "out of principle."

"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. Do not decide for us when to cancel flights," he said.

In recent weeks Belgrade voted three times for United Nations' 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 operation."