The European Union boosted security at its Brussels headquarters on Monday even as authorities downplayed reports that jihadists arrested after returning from Syria were planning to target the building.
Belgian judicial officials said materials which could be used to make explosive devices were found in a raid in the Netherlands but stressed there was no proof of a plot to attack the European Commission building.
However extra security personnel were posted at the entrance to the Commission building in the EU district of central Brussels on Monday, checking the security passes of everyone entering, AFP reporters saw.
Belgian authorities confirmed they had made two arrests under anti-terrorism laws as they sought to prevent jihadist fighters or sympathisers with the Islamic State extremist group from carrying out attacks.
Dutch public broadcaster NOS said Saturday said that the two -- a man and a woman of Turkish descent from The Hague -- had been picked up when they arrived in Brussels. The two allegedly had spent time in Syria.
Meanwhile an internal email from Commission vice president Maros Sefcovic, who oversees security at the EU's executive branch said he had decided to increase security in light of the reports.
"Given the current international situation, and as a precaution, I have decided to raise the security level in the Commission buildings. If necessary further security measures will be put in place," the Belga news agency cited the email as saying.
The Commission is the public face of the European Union and it has one of the most high-profile buildings in Brussels, housing several thousand officials and the top brass in charge of the daily running of the 28-nation bloc.
Spokesman Jonathan Todd said the Commission "constantly monitors" the security situation but that Belgian authorities had confirmed there was no specific threat against the building.
"We do not comment on internal security measures taken by the Commission, we never have done in the past and we're not going to start doing so now for very very obvious reasons," he told a daily briefing.
The Brussels federal prosecutor's office said reports that explosives had been found in raids in Belgium were "incorrect".
"However, in one of the searches in the Netherlands, materials were found which could potentially have been used to make an explosive device -- although they were not explosives and it could not be determined if this concerned an explosive device ready to be used," it said in a statement.
Some of the people under investigation were found to be living near EU institutions but there was "no solid evidence that this could be linked to a concrete plan for a bomb attack," it added.
The two suspects remained in detention, the prosecutor's office added.
Belgium, like several European countries, is increasingly worried about its nationals going to fight in Syria and Iraq for fear they will return home battle-hardened and even more radicalised, posing a threat to security.
It is estimated some 400 Belgians may have done so, while about 90 have returned home.