They are scheduled to play Portugal in a friendly at Brussels’ King Baudouin Stadium on 29 March, but the fixture is now in doubt.
The Portuguese Football Federation said: “We’re in contact with the Belgian authorities and we’ll continue to study the situation at this time.”
Belgium have qualified for Euro 2016 in France and will face Italy, Republic of Ireland and Sweden in Group E.
Mbokani was in the DR Congo squad for their Africa Cup of Nations qualifying double-header against Angola on Saturday and next Tuesday.
It is believed he was on his way to Kinshasa from Brussels to join up with his international team-mates.
Injured Belgium and Manchester City captain Vincent Kompany said he was “horrified and revolted” and tweeted: “I wish for Brussels to act with dignity. We are all hurting, yet we must reject hate and its preachers. As hard as it may be.”
European football’s governing body Uefa said it will continue to “regularly monitor the level of risk for the tournament” and its organisational plans.
Uefa said it would take “all necessary measures” to “guarantee a safe and secure tournament”.