Lithuania has rejected a proposed compromise which would have paved the way for talks on an EU-Russia partnership deal to begin.
Lithuania insists that any EU mandate for talks must include assurances on energy supplies and other issues.
Any EU member state can prevent talks between the entire 27-nation bloc and third countries taking place.
The EU hopes that discussions on the pact with Russia, blocked since October 2006, can be launched at a June summit.
The pact is due to involve energy, migration and other issues. EU foreign ministers may return to the partnership issue when they meet in Luxembourg on Tuesday.
Lithuania, a former Soviet republic, complained that not all of its concerns had been addressed by the EU presidency, currently held by Slovenia.
"We believe that not all our remarks were taken into account, so negotiations have to continue [within the EU]. We cannot accept this proposal," said Violeta Gaizauskaite, a spokesperson for the country's foreign ministry.
Diplomats reportedly attempted to meet Lithuania's demands, which centre on safeguards over a pipeline which transports oil from Russia through Ukraine and Belarus to Europe.
Lithuania has also expressed concern about Russian policy towards Georgia and Moldova.
The EU-Russia negotiations have been stalled since late 2006, when Poland blocked the mandate after Russia had banned meat imports from Poland.
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