Russia demands an official explanation from its ally Serbia over reports that the Balkan country has delivered thousands of rockets to Ukraine for its fight against Russia's invasion.

As AP reports, Russian Foreign Ministry spokeswoman Maria Zakharova expressed "deepest concern" about the reports, which first came from pro-government Russian media last month.

"We are following this story," Zakharova said in a statement posted on the Russian Foreign Ministry website late Thursday. She added that the possible arming of Ukraine represented a "serious question" for Serbian-Russian relations.

According to media reports, state arms factory recently delivered some 3,500 missiles for the Grad multiple rocket launchers used by both the Ukrainian and Russian armed forces. The 122 mm rockets were allegedly transported to Ukraine via Türkiye and Slovakia.

Serbian Defence Minister Miloš Vučević has denied the country exported the missiles to Ukraine but left open the possibility they could have gotten there via a third party.

"If private companies buy weapons in third states' markets and then sell them to other companies in other countries, that is not a question for Serbia, that is international trade," he said.

Serbia is Moscow's closest ally in Europe, with historic, religious and cultural ties bolstered by decades of pro-Russian propaganda campaigns in the Balkan country.

Russia backs Serbia's claim over its former province of Kosovo, which declared independence in 2008 with Western support. And Serbia has refused to impose sanctions on Moscow over the invasion.

Western officials fear Russia could use simmering tensions in Kosovo to try to destabilise the Balkans and avert some attention from the invasion of Ukraine.

Previously, Aleksandar Vučić, President of Serbia, announced that he will take measures against the Serbs (Ukr) who joined the private military company "Wagner" and went to war in Ukraine.