Serbia this week summoned its ambassador in Kiev for consultations after Ukraine raised concerns over Serbian "mercenaries" fighting on the side of pro-Russian rebels in its conflict-torn east.
A foreign ministry statement stressed that Serbia "respects Ukraine's territorial integrity" and had undertaken "a series of concrete moves" against citizens fighting abroad, including in Ukraine.
The announcement on Wednesday followed a release from Kiev a day earlier after Ukraine's Foreign Minister Pavlo Klimkin held talks with his ambassador to Serbia.
The consultations in Kiev focused on "the problem of Serbian mercenaries who fight... as part of the Russian terrorist troops," in the war-torn eastern region, said a statement in English from Ukraine's foreign ministry.
It referred to "Serbia's obligations within international anti-terrorist legal instruments".
"Besides, Ukraine is seriously concerned about illegal contacts by representatives of Serbia with the occupied Crimea," the statement said.
"We hope that the Serbian side will fully commit itself to the principle of respect for sovereignty and territorial integrity of Ukraine within its internationally recognised borders."
The conflict between Russia-backed fighters and Ukrainian troops in the former Soviet republic has killed more than 10,000 people since 2014, when pro-Russian forces occupied parts of eastern Ukraine and Russia annexed Crimea.
The Ukrainian security service told AFP that "more than 300 Serbs" have fought against Kiev troops in the self- proclaimed Lugansk People's Republic alone since the beginning of the conflict, but did not give figures for the rebel stronghold of Donetsk.
But a Ukrainian defence ministry source said it thought these figures were inflated and that "less than 300 Serbs" were estimated to have fought in both self-proclaimed eastern republics.
The Serbian government did not respond to AFP's request for its own estimates of how many citizens had fought in Ukraine.
In Wednesday's statement, Serbian Foreign Minister Ivica Dacic referred to "Ukrainian mercenaries" who took part in Croatian forces' crimes against Serbs during the 1990s Balkan conflict -- "which Ukraine, unlike Serbia, has never condemned".
Serbia "will not allow itself to become a collateral victim of international relations which have nothing to do with Serbia", said the statement.
Russia is often perceived as a big brother figure to Serbia, a fellow Slavic and Orthodox Christian nation.
Serbia refused to follow the decision of the European Union -- which it aspires to join -- in sanctioning Moscow over the Ukraine crisis.
Neither Moscow nor Kiev recognise the sovereignty of Kosovo, a former Serbia province that unilaterally declared independence in 2008.
Belgrade in 2014 amended its criminal code to approve jail terms for citizens who fought in foreign wars. Dozens from Muslim-majority regions also left in recent years to join jihad in Syria and Iraq.
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