Serbian PM Aleksandar Vucic has cut short a visit to Brussels, accusing Croatia of blocking his nation's progress towards EU membership.
Serbia's anger at Croatia - which joined the EU in 2013 - contrasted with the warmth Serbia afforded Russian Foreign Minister Sergei Lavrov on his visit to Belgrade on Monday.
Mr Lavrov was told that Serbia would not seek to join Nato, nor would it join EU sanctions against Russia.
Russia is re-equipping Serbian forces.
The details of new deliveries of Russian military hardware will be discussed in Moscow with Mr Vucic on 21 December. Four MiG-29 jets are expected to be in the package for Serbia.
"If Croatia is the one to decide if Serbia becomes an EU member, then I have suddenly lost interest," Serbian Foreign Minister Ivica Dacic said at a joint news conference with Mr Lavrov.
Serbia gave Mr Lavrov a sabre as a gift for the family of a Russian pilot killed in the Syria conflict.
Deep scars remain from the fighting between Croatia and Serbia in the 1990s, during the break-up of Yugoslavia.
Many Serbs also remain hostile to Nato because of the alliance's bombing of their country in 1999. That bombing was aimed at halting a brutal Serb crackdown on ethnic Albanians in Kosovo, where guerrillas were fighting for independence.
Serbia and Russia are centuries-old allies, sharing an Orthodox Christian bond.
So far Serbia has opened four chapters in its EU accession negotiations, out of 35 chapters in the EU acquis, the body of law applied throughout the bloc.
There will be an accession conference in Brussels on Tuesday and Serbia is due to open two more chapters, a source close to the talks told the BBC.
"They were hoping for a third, but there was no unanimity for that," the source said. Croatia blocked Chapter 26, covering education and culture, because of concerns about Serb treatment of its Croat minority.
Mr Dacic also criticised Bulgaria, suggesting that it had backed Croatia's objection.
Plots and chocolates
Croatian Foreign Minister Davor Ivo Stier reportedly raised concerns about translations of Serb textbooks into Croatian. The two languages are very close - in former Yugoslavia people spoke Serbo-Croat.
"Serbia was patient so far, but now it will talk differently," Mr Vucic said.
In Brussels, Serbian media reported, Mr Vucic "stated his opinion about the behaviour of the countries in the region". He reportedly said he had "had enough" of Croatia's behaviour.
Last week Croatian President Kolinda Grabar Kitarovic apologised for handing out Serbian-made chocolates to children in Dubrovnik.
Mr Dacic said Croatia's leaders had "a problem with chocolates from Serbia" and that the pro-Nazi Ustasha movement was being revived by Croat nationalists.
Neighbouring Montenegro has progressed further than Serbia towards EU accession. It broke away from Serbia in 2006.
Montenegro - also a candidate for Nato membership - suspects Russian agents of plotting to assassinate pro-Western Prime Minister Milo Djukanovic.