Belgian investigators said the two men killed on Thursday night and a third whom they arrested in the town of Verviers were not from area but were using the apartment as a safe house.
They say they found military weapons, including four Kalashnikov AK-47 rifles, hand grenades, explosives, fake documents, communication equipment, substantial sums of money and police uniforms. They believe the three were planning attacks on police in a number of places in Belgium.
In a total of 12 raids in Belgium yesterday, 13 people were taken into custody. Half were in the Molenbeck area of Brussels, where police said they found a number of handguns with ammunition.
Two more, said to have Belgian nationality, were arrested in France at the request of the Belgian authorities, while another two were arrested in Germany.
The authorities said the two men killed in Verviers had spent time fighting in Syria.
Belgian authorities were also questioning a man they believe may have supplied weapons to Amedy Coulibaly who murdered four people in a Jewish supermarket in Paris last week.
Europol spokesperson Soren Pedersen said it had been in close contact with the Belgian authorities, helping in their investigations of the Verviers setup.
A man who took hostages in a post office close to Paris gave himself up to police and nobody was injured. The 25-year-old, arrested during the shoot-out in Verviers, denied having anything to do with terrorism when he appeared before an investigating judge yesterday. He also denied he had spent time in Syria. Several of his relatives were also taken into custody.
Meanwhile, US President Barack Obama and British Prime Minister David Cameron vowed yesterday to take on “the poisonous ideology” of Islamic extremists and said intelligence agencies must be allowed to track militants online, despite privacy concerns.
Both held two days of White House talks amid increasing concern in Europe about the threat posed by extremists after 17 people were killed in Paris attacks and Belgian security forces engaged in a firefight.
“We face a poisonous and fanatical ideology that wants to pervert one of the world’s major religions, Islam, and create conflict, terror and death. With our allies, we will confront it wherever it appears,” said Cameron .
Obama said he and Cameron accepted intelligence and military force alone would not solve the problem, and they would work together on “strategies to counter violent extremism that radicalizes recruits and mobilizes people, especially young people, to engage in terrorism”.
Extremists’ ability to communicate online presented a challenge to authorities, with Obama and Cameron expressing concern about new encryption products that could prevent governments from tracking extremists poised to attack.