Nine suspected terrorist plotters were arrested in Germany and the Netherlands on Thursday accused of planning an ISIS-inspired attack.
The suspects come from former Soviet republics in Central Asia and some arrived in Germany from Ukraine after the outbreak of war, prosecutors said.
The group's alleged plans were deemed "serious enough to intervene" after intelligence agents took up the case.
Dutch police said one suspect, a 29-year-old from Tajikistan, was apparently "given the order to plot a terrorist attack". His Kyrgyz wife, 31, was also arrested.
German authorities said some suspects were in contact with ISIS-Khorasan Province, an affiliate of the terrorist group, and had transferred money to the militants.
The seven arrested in Germany "share a radical Islamic ideology" and formed a terrorist group "with the objective to commit headline-grabbing terrorist attacks in the spirit of ISIS", prosecutors said.
"For the purpose of executing their plan, the suspects had already contemplated targets in Germany, scouted potential crime scenes and attempted to procure weapons."
However, there was not yet a "tangible terrorist plot", authorities said.
At least seven of the suspects are said to have left Ukraine early last year. European countries relaxed border rules to allow mainly women and children to flee the war with Russia.
The G7 countries warned last autumn of a possible security threat from fighters leaving the battlefield in Ukraine.
The seven arrested in Germany were named as Ata A from Turkmenistan, Abrorjon K from Kyrgyzstan and five Tajik citizens Mukhammadshujo A, Nuriddin K, Shamshud N, Said S and Raboni Z.
The affiliate also known as ISIS-K is known for its activities in Central Asia, including in Afghanistan where it opposes the country's Taliban rulers.
Its alleged supporters formed a terrorist cell in Germany in June last year and some had been collecting funds for ISIS since April, authorities claim.
The couple arrested in the Netherlands have yet to be identified. The woman is suspected of preparing terrorist offences but not of being an ISIS member.
The German arrests took place in the state of North Rhine-Westphalia near the Dutch border. All seven men are suspected of supporting ISIS and forming a terrorist organisation.
Judges are expected to decide by Friday whether the group will be held in custody.
