Belgian authorities have confirmed two people were killed and another wounded in an anti-terror raid in the town of Verviers.
At a news conference, a magistrate said the suspects were planning a "major, imminent attack."
Belgian officials said they had been watching the suspects since they returned to Belgium from Syria. Authorities do not believe the suspects had any links to the recent attacks in Paris.
The suspects were reportedly shot after they first opened fire on authorities near a train station. No casualties among law enforcement have been reported.
"They suspect that this group ... were directed by ISIS to launch a terrorist attack in Belgium, a terrorist attack in Europe, in retaliation for the air strikes in Syria and Iraq," Paul Cruickshank said.
Belgium's terror alert has now been raised to its second-highest level, and officials say anti-terrorism campaigns are being carried out in other parts of the country.