VILNIUS – Lithuanian President Gitanas Nauseda said in Brussels on Thursday that it was difficult to predict when Wagner's mercenaries might turn "against us".
Nauseda said it was "very likely" that Wagner's leader Yevgeny Prigozhin was already in Belarus.
"It is very likely that he is already there. The fighter groups of these serial killers could be and emerge in Belarus at any moment and nobody knows when they could turn against us," he told reporters ahead of the European Council's meeting.
The president said he disagreed with the opinion of some other leaders that a strong Russian President Vladimir Putin is less dangerous than a weak one.
"We have to move forward and to be decisive, because now it is a crucial moment of history. If we are not decisive and united now, tomorrow it will be too late,"
Wagner's forces took control of military sites in the southern Russian city of Rostov-on-Don last weekend and threatened to overthrow the country's military leadership, but eventually abandoned their march on Moscow.
The Kremlin then announced that Wagner's leader Yevgeny Prigozhin would leave for Belarus and that the group's mercenaries had also been allowed to retreat to the country.
Nauseda also said that the continuation of military support to Ukraine, and security guarantees would be a key issue on the agenda of the European Council's meeting in Brussels.
"Ukraine is doing very well in implementing reforms and President (Volodymyr) Zelensky told (us) a lot about the success of legal systems and anti-corruption measures," he told reporters.
Nauseda also said that he expected talks on Ukraine's EU membership to start in December. The EU granted Ukraine candidate status last year.
Moldova, Ukraine and Georgia deserve "positive results" if they successfully carry out reforms, according to him.
In Brussels, Nauseda also called on the EU to start discussions on a 12th package of sanctions against Russia.
The 11th package, adopted earlier in June, aims to close the loopholes for circumventing the existing sanctions against Moscow over its invasion of Ukraine.
In the president's words, the 11th package is "a very important step, but not a big one".
"This is the reason why we expect a twelfth package of sanctions. I will not be tired to repeat that Rosatom should be punished because it is just unacceptable what they are doing," he said.