His Highness the Emir Sheikh Tamim bin Hamad al-Thani and Nato Secretary General Jens Stoltenberg witnessed the signing of the agreement between the Government of Qatar, represented by the Qatari Armed Forces, and Nato.
The Prime Minister of Belgium, Charles Michel, welcomed His Highness the Emir and the official delegation, expressing his delight at the Emir's visit to Belgium.
A Belgian judge made no ruling on Friday on a European arrest warrant issued by Spain for ousted Catalan leader Carles Puigdemont, his lawyer said after a court hearing in Brussels.
Security checks for passengers boarding high-speed trains have been increased at three major Belgian stations since last month.
Belgian prosecutors said on Wednesday they had charged two Belgian nationals with terrorist activity after police found weapons and detonators during house searches overnight in the capital Brussels.
Belgian authorities on Wednesday identified a Moroccan man behind a foiled nail bomb assault on a busy Brussels train station, the latest in a wave of attacks to hit Europe.
Belgian counter-terrorism police are investigating the identity of a suspected suicide bomber shot dead by troops guarding a Brussels railway station after he set off explosives that failed to injure anyone.
Belgian troops patrolling Brussels Central Station "neutralised" a person after a small explosion on Tuesday, a police spokesman said, adding that there were no other casualties and the situation was under control.