Here are some facts about the Organisation for Security and Cooperation in Europe and its current activities in the former Soviet republic.
Cold War beginnings
The Vienna-based OSCE saw the light of day in the early 1970s as a forum for dialogue between East and West.
After the end of the Cold War, it remained a significant player in the former Soviet bloc, where it helped to monitor so-called frozen conflicts in Nagorno-Karabakh and Transdniestr, as well as the 2008 conflict in South Ossetia.
But it was with the crisis in Ukraine that the OSCE's star gained prominence in the West.
It now counts 57 member states on three continents -- including Russia, Ukraine and the United States -- as well as 11 partner nations. In January, Serbia took over the organisation's one-year rotating chairmanship.
The OSCE's activities include election monitoring, conflict prevention and resolution, building democratic institution and training police. It currently has missions in around 15 countries, primarily the former Yugoslavia and central Asia.
The OSCE's "Special Monitoring Mission" (SMM) in Ukraine -- in place since March 2014 -- currently counts 412 international monitors from 41 countries, including the US, Finland, Switzerland and Russia.
Over half of them are deployed in the main rebel-held regions of Donetsk and Lugansk. Their job is to gather information on the ground, investigate incidents and facilitate dialogue among the various partners.
SMM members were the first international observers at the site of the MH17 plane crash in eastern Ukraine in July 2014.
The OSCE also deployed a military verification mission in March 2014 "to dispel concerns about unusual military activities", with about 50 military and civilian staff from various member states.
In late July, another observer mission of about 20 monitors was set up at two Russian-Ukrainian border checkpoints -- Gukovo and Donetsk.
The OSCE has also sent election observers as well as special representatives on national minorities and press freedoms to assess the situation in Ukraine since the start of the crisis last spring.
Under a new peace roadmap, agreed on Thursday by Kiev and pro-Russian rebels, the OSCE will monitor the withdrawal of heavy weapons and troops from the frontline, as well as the withdrawal of "all foreign armed formations, military equipment, as well as mercenaries from the territory of Ukraine".
The Vienna-based organization will also be tasked with monitoring local elections in separatist regions at an as-yet unspecified date.
The roadmap, agreed after marathon negotiations in Minsk involving the leaders of France, Germany, Russia and Ukraine, calls for a ceasefire to come into force at 2200GMT on Saturday (midnight Kiev time).
A previous truce signed last September in Minsk by the warring parties and the OSCE quickly collapsed.