Russia has been accused by Lithuania of violating international maritime laws after seizing a Lithuanian fishing boat with a crew of 30.
The EU protested earlier over the 18 September incident, saying Russian border guards had conducted a "forced apprehension" of the vessel.
Russia says the vessel, Juros Vilkas, was illegally fishing for crabs - a claim denied by Lithuania.
It is now in the Russian port of Murmansk awaiting a court ruling.
The incident is the latest involving alleged Russian provocation of its Baltic neighbours.
The seizure comes at a time of heightened tensions between Russia and the EU.
Relations have been strained ever since Russia's annexation of Crimea from Ukraine in March, after a controversial referendum in the region.
The EU responded with a series of sanctions targeting Russia's oil interests, defence firms and banks.
Court to rule
Rasa Jakilaitiene, a spokeswoman for Lithuania's foreign ministry, told reporters the Juros Vilkas had been in international waters when it was seized in the Barents Sea. She said that assessment was supported by satellite data.
A spokesman for the EU said earlier: "The European Union calls on Russia to respect its international obligations and to immediately release the vessel."
Officials had raised the issue with the Russian ambassador to the bloc, he added.
Russia's foreign ministry said the fishing boat had been detained after guards found it in the country's exclusive economic waters - an internationally recognised area of sea with rights for a particular nation.
The Lithuanian government acknowledged the boat had been in an agreed Russian fisheries zone but insisted it had a right to be there.
A Russian court will decide on 7 October whether to release the vessel, AFP news agency reports.
Lawyer Vladimir Odiagailo told the agency that the crew, which includes Lithuanian and Russian nationals, had "not been detained and is still on the boat, anchored in the port of Murmansk".
In early September the Estonian government said a security officer had been "abducted" on the country's border and taken into Russia.
Moscow's intelligence agency countered that the man was a suspected spy and had been detained on its own soil.