Belgium has been put on edge over potential Islamist terrorist attacks for the second time in four months amid reports that a man and woman who had returned from the war in Syria via Turkey were plotting an assault on the European Union's main offices in Brussels.

According to the Dutch public broadcaster, NOS, the couple were detained in Belgium where investigators were said to have proof of a planned attack on the Berlaymont building, the offices housing the European commission.

A commission spokeswoman said she was aware of the reports, but that the organisation, the EU executive and civil service, had received no warnings of any specific threats.

With governments across Europe increasingly preoccupied by the risks arising from the return of nationals who have joined gone to Syria and Iraq to join Islamic State (Isis), concern is particularly high in Belgium.

Four-hundred Belgians are said to have travelled to Syria to join the extremists, usually via Turkey and its long, porous border into the war zone. While that figure is much lower than the estimates for Britain, France or Germany, proportionately and in per capita terms Belgium is believed to have the highest number in Europe of would-be jihadists travelling to Syria.

The Dutch and Belgian security services have carried out coordinated raids on suspected returning fighters and sympathisers over the past month. The couple, believed to be resident in The Hague in the Netherlands and of Turkish origin, were said to have been arrested at Brussels airport after returning from Syria via a flight from Turkey.

The Guardian's Brussels office is adjacent to the European commission building and there was no evidence of beefed up security measure in place in the district on Sunday.

According to the Belgian newspaper L'Echo, almost a quarter of the suspected Belgian jihadists have returned home from the Middle East. It quoted national counter-terrorism investigators as saying that at least 10 of those who had returned were plotting attacks on their home territory.

In June, 46 suspected members of Sharia4Belgium, a radical Islamist group believed to be involved in sending young fighters to Syria, were ordered to face trial on charges that include belonging to a terrorist organisation.

A French-Moroccan man is under arrest in Belgium awaiting trial after allegedly spraying the Jewish Museum in central Brussels with automatic gunfire in May, killing four people.

Mehdi Nemmouche is awaiting trial on "terrorist murder" charges. He had spent more than a year fighting with Islamist extremists in Syria, according to authorities in Belgium and France where he was arrested.

A fire at a synagogue in the Brussels district of Anderlecht last week is believed to have been the work of arsonists.

According to the Dutch broadcaster, citing Belgian investigators, the couple may be charged with belonging to a terrorist organisation, of breaking laws on possession of weapons, and of funding terrorism.