Belgian police arrested six people in their probe of Tuesday’s Islamic State suicide bombings in Brussels, while authorities in France said they thwarted a militant plot there “that was at an advanced stage.”

The federal prosecutor’s office in Belgium said on Thursday that the arrests came during police searches in the Brussels neighbourhoods of Schaerbeek in the north and Jette in the west, as well as in the centre of the Belgian capital.

The arrests came days after suicide bombers hit the Brussels airport and a metro train, killing at least 31 people and wounding some 270 in the worst such attack in Belgian history.

The attack in Brussels, which is home to the European Union and NATO, has heightened security concerns around the world and raised questions about European countries’ response to the threat from Islamist extremists.