BRUSSELS - Belgian police have arrested two people suspected of plotting attacks in Brussels during New Year celebrations, just weeks after the militant bombings and shootings in Paris which were allegedly planned in Belgium.

The federal prosecutor's office in Brussels, the home of the European Union and NATO, said Tuesday that police seized military-style training uniforms, computer hardware and Islamic State propaganda material in raids in various parts of the country.

But investigators said the police action on Sunday and Monday was not linked to the wave of deadly attacks in Paris in November which were claimed by the Islamic State group and which France says were prepared in Belgium.

One of the two was arrested on suspicion of planning attacks as well as "playing a lead role in the activities of a terrorist group and recruiting for terrorist acts," the prosecutor's office said in a statement.

The second faced charges of planning and "participating in the activities of a terrorist group," it said.

"The investigation cast a light on serious threats of attacks believed to be aimed at several emblematic sites in Brussels and carried out during the end-of-year celebrations."

In light of the "serious" threats, Belgium's OCAM national crisis centre late Monday raised its alert level for police and soldiers in Brussels, "which could be symbolic targets," a spokesman told AFP.

In the last year, the Belgian authorities have deployed troops in addition to police reinforcements outside many locations in Brussels, including European Union buildings and foreign diplomatic missions, as fears of militant attacks have grown.

The two new suspects were arrested during raids in the Brussels area, in the Flemish Brabant area to the north of the capital and near Liege in the eastern part of Belgium's southern French-speaking region of Wallonia.

The raids, which were ordered by an investigating magistrate in Brussels who specialises in terrorism cases, turned up neither weapons nor explosives.

A total of six people were detained, including the two suspected of plotting attacks, but the four others were later released, the prosecutor's office said.

It said investigators were examining seized computer hardware, uniforms and Islamic State propaganda material but declined to release any details about the suspects as the investigation was ongoing.

Prime Minister Charles Michel was following developments closely but did not have any immediate plans to make a statement, his office told AFP.

The Belgian authorities are still looking for suspects linked to the November 13 attacks on a Paris concert hall, restaurants, bars and the national stadium which left 130 people dead and hundreds more wounded.

The top fugitive is Brussels-born Salah Abdeslam, 26, who is suspected of having played a key role in the Paris carnage and understood to have returned to the Belgian capital the day after the bloodshed.

An international arrest warrant is out on Abdeslam, who lived in the troubled Brussels district of Molenbeek, which analysts say has served as a haven for militants.

Nine men have been detained including four accused of helping Abdeslam get away in the hours after the attacks.

Since the end of November, Brussels has remained at alert level three, one notch below the maximum alert of a serious and imminent terrorist threat. For four days before then, officials fearing a repeat of the Paris attacks closed schools and underground train service as they put Brussels on maximum alert.

Belgium's Interior Minister Jan Jambon told Tuesday's edition of Le Soir newspaper that Abdeslam has been able to evade capture for so long because he has "support in the communities" and expressed surprise at the level of such support.

Belgium is per capita the source of the highest number of foreign fighters in Syria and Iraq with an estimated 500 of its citizens having gone to wage jihad there.

One of them is the alleged Paris attacks mastermind Abdelhamid Abaaoud, who was killed in a police raid a few days after the massacre.