Security services across Europe have uncovered an ISIS plot to use explosives to target churches.
Counter-terrorism experts say the group remains a “serious threat” and is taking advantage of the conflict in Gaza to motivate its supporters.
Over the Christmas period, police have made several arrests in Spain, Austria, France and Germany.
In Germany, police fear terrorists are plotting an attack on Cologne Catherdral and have increased security.
They are now carrying out strict controls on people entering the cathedral following a “danger warning” for New Year's Eve.
Austrian police have also stepped up checks around churches, religious events and Christmas markets in Vienna, citing an “increased risk”.
“These arrests demonstrate once again that ISIS remains a serious threat to the West,” Counter Extremism Project chief executive Ambassador Mark Wallace said.
“ISIS is not only trying to take advantage of Hamas’s war against Israel to motivate its supporters and sympathisers but is also attempting to organise terror attacks in Europe to demonstrate its continued relevance within the terrorist milieu.
“Authorities must continue to increase their co-operation to ensure the futility of these efforts.”
Austria on Sunday said three people were detained for suspected involvement in an “Islamist network”.
However, the interior minister said there was “no immediate threat” of an attack in Vienna.
“Terrorist actors across Europe are calling for attacks on Christian events,” Vienna police said.
One person has been arrested in Germany and the country's Interior Minister Nancy Faeser has pledged a firm response to any extremist threat.
“We all love our Christmas traditions and won't allow ourselves to be intimidated or have our way of life hemmed in,” she said.
“Our security authorities have their sights on the Islamist scene and are acting decisively as the current measures show.”
Spain also received indications that an Islamist group was planning several attacks in Europe, possibly on New Year's Eve.
Last week, five people were apprehended in northern France as part of an investigation into a criminal terrorist conspiracy.
They had been arrested following fears that an attack was being planned.
CEP senior director Dr Hans-Jakob Schindler said Afghanistan continues to be an active terrorist zone and, despite the war in Gaza, nations need to ensure the situation there is not ignored.
“Afghanistan remains a very active terrorist zone. The Taliban regime is not only extremely repressive domestically, violating all basic human rights including women’s rights, but also maintain their symbiotic relationship with al-Qaida and its various affiliates in the country,” he said.
“In addition, the Taliban are not a reliable partner in the fight against ISIS in Afghanistan. Therefore, rather than further relaxing the control mechanisms that are deployed to ensure that humanitarian deliveries are not diverted to terrorist groups in Afghanistan, as some propose, strict monitoring structures continue to be necessary to ensure that finances flowing into Afghanistan are not diverted. Ignoring the situation in Afghanistan will only come at our collective peril.”
