Ukraine said on Wednesday it had detained two Islamic State terrorists from Syria who were trying to cross into western Europe in order to stage new attacks.
The SBU national security service said the cell was uncovered in the northeastern city of Kharkiv, which has remained under government control throughout the ex-Soviet republic's 25-month separatist war.
"The Kharkiv national security service of Ukraine has uncovered an attempt to send supporters of the Islamic State international terrorist organization from Syria by transit through Ukraine to western European states," the SBU said in a statement.
It was not clear from the statement when the arrests were made.
"The organizer of this illegal transfer was a citizen of a neighboring country who is living unlawfully in Ukraine and is suspected by law enforcement officials of possible involvement in terrorist activity," SBU said.
A senior source told AFP on condition of anonymity that the suspect assisting the ISIS group in Ukraine was a Russian national.
"Two of the Islamic State supporters detained in Kharkiv were Syrian nationals," SBU spokeswoman Olena Gitlyanska to AFP by telephone.
The security service said it could not provide further details because an investigation was still underway.
It was not immediately clear which western European countries the ISIS members intended to attack.
Europe has been on heightened alert since last year's jihadist attacks in Paris and subsequent bombings in Brussels.
The United States warned its citizens on Tuesday that the upcoming Euro 2016 football championship and related events across France and Europe would present "potential targets for terrorists."
AFP contributed this report.