Brussels has warned Serbia it must dissolve a proposed new free-trade deal with Russia if it wants to join the European Union.
The Balkan country said it planned to join the Moscow-led Eurasian Economic Union (EAEU) in October.
But a European Commission spokesperson told Euronews that Serbia would have to withdraw from all existing bilateral free trade agreements before joining the bloc.
The commission's comments came after a meeting between Serbian President Aleksandar Vucic and Russian Ambassador Alexander Botsan-Kharchenko, who said the deal would grant Serbia access to a market with a capacity of over 180 million people.
The EU's single market in comparison currently comprises over 500 million consumers, worth €17 trillion
Botsan-Kharchenko said in a tweet that “this is a significant event" which "the media is paying due attention to”.
The EAEU, which was established in 2015, includes Russia, Armenia, Belarus, Kazakhstan, and Kyrgyzstan as members.
Serbia is a longtime ally of Moscow but to join the bloc, countries are required to align their policies with the EU's, which would include imposing trade sanctions on Russia over its actions in Ukraine.
It applied to become an EU member in 2009, becoming an official candidate in 2012, with talks beginning in 2014 but little progress has been made since then.
Some in Belgrade fear the country won't become an official member until 2025.
Both Serbia and Kosovo were told to resolve their differences to resume membership talks with the EU.
Talks between the two countries broke down in November when Kosovo slapped a 100% tax on Serbian and Bosnian goods to retaliate against them not recognising the land-locked territory as a country.