Russian ambassadors have been summoned to several European foreign ministries amid reports of imminent diplomatic expulsions, reports say.
EU leaders agreed last week it was highly likely Russia was behind the poisoning of an Russian ex-spy and his daughter in the UK.
The bloc recalled its envoy to Moscow.
On Monday, Latvia, Lithuania, Estonia and Poland summoned Russian ambassadors to their foreign ministries, Russian news agencies said.
A source in the Lithuanian foreign ministry confirmed to the BBC that it had called in the ambassador in Vilnius. There were similar reports from Estonia and Poland.
No official reason has so far been given for the summons but a number of countries across the EU had been expected to expel Russian diplomats this week over the poisoning of former Russian spy Sergei Skripal and his daughter Yulia on 4 March in Salisbury, southern England.
Russia has denied outright any connection to the attack. The pair remain in a critical but stable condition in hospital.
European Council President Donald Tusk said at the end of an EU summit on Friday that more than one country would expel diplomats “but I don’t think that it will be the whole group”.
Britain expelled 23 Russian diplomats last week who it said were spies, in the aftermath of the nerve agent attack. Russia has responded with tit-for-tat expulsions.
US President Donald Trump is also weighing up whether to expel Russian diplomats, as recommended by the National Security Council, US media report. Kremlin spokesman Dmitry Peskov has threatened to respond in kind.
On Friday, French President Emmanuel Macron called the use of the Soviet-era Novichok agent in Salisbury an “attack on European sovereignty”.
German Chancellor Angel Merkel said co-ordinated measures among EU countries in response would be “necessary”.