Photos, which began circulating on social media and defence blogs earlier this week, showed missiles believed to be Chinese-made CM-400AKG cruise missiles attached to Serbia's Russian-made MiG-29 planes.

"We have things we do not show," President Aleksandar Vucic told Serbia's national broadcaster when asked about the rockets.

"We have a significant number of those missiles, and we will have even more," he added.

He declined to give further details but said they were "extremely expensive" and "extremely effective".

Purported to be the export version of China's YJ-12, experts believe the missiles can travel close to hypersonic speed.

Serbia, an EU candidate, is one of the few countries in the Western Balkans that is not a NATO member and has invested heavily in its military to "preserve neutrality".

Its latest publicly known purchase was an order of 12 French-made Rafale fighter jets worth 2.7 billion euros ($3.1 billion).

Belgrade also maintains close ties with Moscow, from which it has previously purchased weapons.

In a region torn apart by war in the 1990s, images of the Serbian jet armed with the new missiles drew attention from its former Yugoslav neighbours.

Croatian Prime Minister Andrej Plenkovic said he would warn NATO about "such weaponry, which is new in the arsenal of the Serbian army".

Vucic said his country has good relations with the Western military alliance and "will do everything" to maintain them.

"Everything we do is to defend our country," Vucic added.