Belgian troops are still highly visible in Brussels following the March terrorist attacks | Photo credit: Press Association
Europe’s anti-terrorism chief Gilles de Kerchove also said it was likely newly-radicalised men and women would return to Europe from Syria and Iraq "sooner than expected."
His warning comes after Belgian authorities this week charged three men with terrorism offences, including attempting to commit murder, following a large-scale operation at the weekend.
Belgian media reported that militant Islamists may have been planning to attack the football fan zone in Brussels, where Belgium's games in the Euro 2016 tournament in France are screened.
De Kerchove told a meeting in Brussels on Tuesday (21 June), that Islamic State (IS) was "on the defensive" after losing key territory in Syria and Iraq.
The terror group has intensified its attacks in Baghdad as the Iraqi army continues a major offensive to try to drive IS from its stronghold of Falluja, 60km west of the capital.
IS still controls large swathes of territory in the country's north and west, including Mosul, Iraq's second largest city.
The group is also under pressure in neighbouring Syria, where it has been targeted by government forces and US-backed rebels.
De Kerchove said, "IS has recently lost a lot of territory where it might have expected to extort money from local citizens but there are consequences to this.
"As this is an organisation that needs to show it is succeeding, more plots can be expected, though not necessarily actual attacks, in the west.
"Another consequence of this loss of territory are the so-called foreign fighters who have left for Syria and Iraq but who could now start to return to Europe sooner than we expected. That presents an obvious risk as well."
He also cautioned that while it was IS that was behind the Paris and Brussels atrocities, it would be a "big mistake" to overlook the continued threat of Al-Qaeda.
The Belgian-born official said the west faced a threat both from "homegrown" terrorists, who were radicalised via the internet, and "foreign fighters" who leave to fight in Syria and Iraq.
Europe and the west, he argued, had to learn the same lessons and how to "connect the dots" as the Americans had done after 9/11.
The "good news", he said, was that the west was "better mobilised than ever before" in its readiness to prevent terror attacks.
Even so, the international community still had to "be smarter" in prevention and counter terrorism and also in tackling the forces behind radicalisation.
Addressing a debate on 'Security v. Liberty', organised by the Friedrich Naumann Foundation for Freedom and the AJC Transatlantic Institute, De Kerchove said it was also vital to "strike the right balance" between combating terrorism and privacy issues.
He cited the example of efforts to outlaw terror-related material from the internet, saying that while he supported this, it should not come at a loss of free speech.
The counter terrorism coordinator said, "Removal of offensive online content raises some important issues. For example, we have to make the distinction between content that is unlawful and that which is distasteful.
"While we may not agree or like distasteful material, that is not necessarily the same as content that is unlawful."
He also cautioned against cultivating a climate of "Islamophobia" and "ostracising certain communities."