The United Nations Iran-Iraq Military Observer Group, abbreviated as UNIIMOG, was responsible for supervising the ceasefire between Iran and Iraq.

On 9 August 1988, the UN Security Council adopted Resolution 619, by which UN military observers under the name UNIIMOG would be stationed around the Iran-Iraq borders. With trilateral negotiations among Iran, Iraq and the United Nations, the forces of both sides returned to international borders and the ceasefire was established from 6:30 AM on 10 August 1988.⁠[1]

On 10 August 1988, UNIIMOG forces entered Iran and Iraq for 6 months. Confirming, consolidating and supervising the ceasefire, supervising the withdrawal of forces to recognized international borders, assisting the parties involved in resolving issues, especially regarding precise determination of border lines, and assisting them in creating a buffer zone along the borders of the two countries were among the duties of UNIIMOG.⁠[2] UNIIMOG included 400 officers from Argentina, Australia, Austria, Bangladesh, Canada, Denmark, Finland, Ghana, Hungary, India, Indonesia, Ireland, Italy, Kenya, Malaysia, New Zealand, Nigeria, Norway, Peru, Poland, Senegal, Sweden, Turkey, Uruguay, Yugoslavia and Zambia⁠[3] and their command, upon proposal of the UN Secretary-General and approval of the Security Council, was assigned to General Slavko Jovic from Yugoslavia.⁠[4] Iran and Iraq provided liaison officers as well as requested logistical facilities for coordination and cooperation with UNIIMOG.

Military observers were stationed in four sectors on the Iranian side with headquarters in Saqqez, Bakhtaran (Kermanshah), Dezful and Ahvaz and three sectors on the Iraqi side, with headquarters in Sulaymaniyah, Baqubah and Basra. Each sector controlled a number of sites located near the border to minimize the distance of observers to the ceasefire line. The length of the ceasefire line supervised by one site varied from 70 km to 250 km and the ceasefire lines in total were 1400 km.

Teams consisting of two or more military observers carried out mobile patrols by vehicle, helicopter, boat in marshes and by mule or on foot in mountains. In winter some patrols also used skis. On average 64 patrols were carried out daily. The main duty of patrols was supervising observance of the ceasefire by Iran and Iraq, and they communicated complaints of their side to their counterparts on the opposite side of the ceasefire line. In addition to investigating alleged violations, military observers also carried out humanitarian actions such as exchange of war dead found on the battlefield.

During the mission, UNIIMOG received repeated complaints about ceasefire violations. In the first 9 weeks, 1072 such complaints were registered, but with stabilization of the ceasefire, this number steadily decreased. However, more significant violations occurred. On 23 August (1 Shahrivar 1367), shortly after the start of the ceasefire,⁠[5] in the Pich-engizeh, Sharhani regions and also north of Fakkah, 666 personnel of the Iranian Army were captured by Iraq.⁠[6]

Among other Iraqi violations were several shootings, movement of forces, creation of new observation posts, strengthening defensive positions with wiring, mining, improving trenches and engineering works. In all these cases, UNIIMOG tried to convince Iraq to stop this and return conditions to previous status.⁠[7]

With the end of the 6-month mission of UN peacekeeping forces, the Security Council adopted Resolution 631 on 8 February 1989, on the basis of which the mission of the UN military observer group was extended for 7 months and 22 days until 30 September (8 Mehr 1368); again on 29 September 1989, it adopted Resolution 642, on the basis of which the mission was extended for another six months until 31 March 1990.⁠[8]

Until the end of September 1990, withdrawal of Iranian and Iraqi forces to international borders was almost complete. In this situation, the UN Secretary-General recommended that the Security Council extend UNIIMOG mission for a limited two-month period instead of six months as done in three previous occasions. At this time UNIIMOG needed only 60 percent of its existing force to perform its duties.

After adoption of Resolution 671 on 27 September 1990, in which the Security Council agreed with the Secretary-General recommendations, UNIIMOG personnel were reduced to 184 military observers. During this reduction, military observers were stationed in three sectors on the Iranian side with headquarters in Saqqez, Bakhtaran (Kermanshah) and Dezful and three sectors on the Iraqi side, with headquarters in Sulaymaniyah, Mansuriyah and Basra.

Iraq preferred UNIIMOG mission to be extended for another full six-month period but Iran refused to accept extension for more than two months. Iran insisted on reducing observers to 50 to 60 persons on each side. In this situation, the Secretary-General recommended extension of UNIIMOG mission for a two-month period with 120 military observers to the Security Council, which the Council accepted on 28 November 1990 with adoption of Resolution 676.

Reduction of military observers to 60 persons on the Iranian side, 56 in Iraq and 3 in the command group required reorganization of UNIIMOG deployment.

With occupation of Kuwait by the Iraqi Army and adoption of Security Council Resolution 678 on 29 November 1990 against Iraq, UNIIMOG activity on Iran-Iraq borders was affected by this war. With deterioration of conditions in Iraq in the second week of January 1991, the number of UNIIMOG personnel in Baghdad gradually decreased. After the start of the war of Iraq against Kuwait on 16 January 1991, the remaining UNIIMOG forces in Iraq were transferred to Cyprus or Iran. However the UN Secretary-General recommended to the Security Council that UNIIMOG mission, which ended at the end of January 1991, be extended for another month so that this group fully performs its responsibilities. On 31 January 1991, the Security Council unanimously adopted Resolution 685 and UNIIMOG mission was extended until 28 February 1991.

In the next month, UNIIMOG continued activity only in the Iranian sector and also had contact with Iraqi authorities on the border.

On 26 February 1991, the UN Secretary-General in a report to the Security Council announced that Iranian and Iraqi forces had fully withdrawn to international borders and Resolution 598 had been implemented. The Secretary-General recommended that UNIIMOG mission not be extended, and thus UNIIMOG mission which had started in August 1988 ended on 28 February 1991.⁠[9]

Leadership of UNIIMOG from August 1988 to November 1990 was held by General Slavko Jovic from Yugoslavia and from November 1990 to February 1991 by Brigadier S. Anam Khan from Bangladesh. UNIIMOG mission cost 177.9 million dollars (for the United Nations).⁠[10]