(CNN) -- Belgian authorities thwarted a suspected terror cell that was about to carry out a major terrorist attack, the federal prosecutor's office said.
Two suspects were killed Thursday in the anti-terrorism operation at a building in the eastern city of Verviers, prosecutor's spokesman Eric van der Sypt said. A third suspect was injured and taken into custody.
A senior Belgian counterterrorism official told CNN that the alleged terror cell received instructions from ISIS.
Some members of the cell had traveled to Syria and met with ISIS, which plotted the attacks as retaliation for U.S.-led airstrikes in Syria and Iraq, the Belgian source said.
The federal prosecutor's office said that at least some of the group members had been in Syria.
"This was in the framework of an operation looking into an operational cell made up of people, some of whom coming back from Syria," van der Sypt said. "The investigation made it possible to determine that the group was about to carry out major terrorist attacks in Belgium imminently."
The trio had been under surveillance for some time, he said.
Verviers is about 69 miles (111 kilometers) east-southeast of Brussels and 200 miles (322 kilometers) northeast of Paris.
Anti-terrorism operations were underway in other cities, the Belgian counterterrorism official said.
The raid comes amid heightened alerts in Europe after this month's attack on the office of satirical magazine Charlie Hebdo in Paris.
A video posted on YouTube showed part of the Belgian operation -- shouts and gunfire can be heard.
"At Verviers, the suspects immediately, and for long minutes, fired using weapons and hand weapons before being neutralized," van der Sypt said.
Countries are on high alert
European security services in recent weeks have received indications that ISIS may have started directing European extremists in Syria and Iraq to launch terrorist attacks back in their home countries, the Belgian counterterrorism official said.
The official said security agencies in several European countries were intensely investigating several groups of returnees from Syria and Iraq, including the group that authorities confronted in Belgium.
There was no suggestion the returnees are connected with the Kouachi brothers, who carried out the attack on Charlie Hebdo, nor Amedy Coulibaly, the man who attacked the Paris kosher supermarket and also pledged allegiance to ISIS.
The official described the move as an apparent significant shift by the terrorist group. Prior to the air campaign against it, they said, there was little indication the ISIS leadership was directly plotting attacks in the West. Instead they said the group prioritized its project to create an Islamic caliphate.
The official named France, the UK and Belgium as countries facing a particular threat. Counterterrorism agencies in Germany are also on high alert because of the number of fighters who have traveled. Several European countries including Britain, France, Belgium and the Netherlands are currently participating in the air campaign against ISIS in Iraq.
The assessment is the shift is partly the result of increased competition between ISIS and al Qaeda affiliates, including the Khorasan group in Syria, to be seen as the standard bearers of global Jihad, according to the official.
The official said there was also significant concern about Khorasan attack plotting against Europe. U.S. officials previously told CNN that French al Qaeda operative and bomb-maker David Drugeon was suspected to be talent-spotting European jihadis in Syria for operations in Europe. Drugeon was injured in a drone strike in November but is believed to be still alive.
Last week, Andrew Parker, the head of Britain's security service MI5, warned that a group of core al Qaeda terrorists in Syria is planning mass casualty attacks against the West," an apparent reference to the Khorasan group.
Shift in strategy for ISIS?
European officials have been warning for months about the unprecedented challenge posed by returning fighters. More than 3,000 Europeans have left to fight in Syria in recent years. The total number who have returned to Europe is now estimated to be over 500, including 250 who have returned to the UK, almost 200 to France and around 70 to Belgium.
Several returning ISIS fighters have already been implicated in attack plans in Europe. In February, police in Cannes broke up an alleged plot to bomb targets in France by Ibrahim Boudina, a French-Algerian extremist who had allegedly just returned from fighting with ISIS in Syria.
Police say they found almost a kilogram of the high explosive TATP inside soda cans in his family's Cannes apartment building. Screws and nails were attached to one with sticky tape as shrapnel, according to sources briefed on the investigation. Boudina has denied the allegations against him.
Mehdi Nemmouche, a French-Algerian ISIS fighter who allegedly helped guard Western hostages in Syria before returning to Europe, allegedly shot and killed four people at a Jewish museum in Brussels in May.
Nemmouche was arrested in France and extradited to face trial in Belgium. He has denied the charges. In both the Cannes and Brussels plots, investigators believe it is possible the men were acting on their own steam.
No evidence has been publicly released suggesting ISIS leadership signed off on the plots. But the worry now is that ISIS has pivoted towards launching attacks in Europe.
European officials say all this adds up to an unprecedented terrorist threat in Europe. Late last year, just weeks before the attacks on a satirical magazine, Jewish grocery and police officers in Paris, French Prime Minister Manuel Valls said France had never faced a greater terrorist threat.
ISIS runs large training facilities in Europe and Syria, has deep financial pockets, and access to thousands of potential European recruits.