The OSCE is taking on an ever more important role in the Ukraine crisis, with both Moscow and the West looking to the international security body to help ease tensions.
Here's what you need to know about the Vienna-based Organization for Security and Cooperation in Europe and its current activities in the former Soviet republic.
Cold War beginnings
Based in Vienna, the OSCE began life in the 1970s as a forum for dialogue between East and West and after the Cold War ended in 1990-91 it took on a broader role.
It is now made up of 57 participating states - including Russia, Ukraine and the United States - on three continents. An additional 11 countries are partner nations. The current chair is Switzerland.
Its activities include election monitoring, conflict prevention and resolution, helping states to develop democratic institutions, training police and assisting in military reforms.
There are currently OSCE activities in around 15 countries including parts of the former Yugoslavia - torn apart by wars in the 1990s - as well as former Soviet republics Tajikistan and Uzbekistan.
In Ukraine, the OSCE has nine different fields of activity including a planned 1,000-strong observation mission for the scheduled presidential election on May 25, a "national dialogue project" and a human rights assessment mission.
There is also a military verification mission, eight members of which were kidnapped by pro-Russian militants in the eastern Ukrainian town of Slavyansk on April 25. One was released two days later and the others only on May 3.
That mission is not however staffed by the OSCE but by military experts from OSCE member states. It was not approved by Russia but was deployed in accordance with the OSCE's so-called Vienna Document agreement of 2011.
Requested by Ukraine, the unarmed mission's aim is to "dispel concerns about unusual military activities". It attempted but failed to enter the annexed peninsula of Crimea four times, with warning shots fired on March 8.
Special Monitoring Mission
The OSCE's principal 'Special Monitoring Mission', which was approved by all states including Russia on March 21 but with no access to Crimea, currently numbers 160 civilian unarmed monitors, supported by local staff, due to rise to 200 by the end of May.
The mission can expand in scope to up to 500 monitors. Their job is to meet with authorities, NGOs, ethnic and religious groups and local communities, gathering facts on the ground and information on the security situation.
Its headquarters are in Kiev, with monitors deployed in teams of ten across the country, including flashpoint areas in the east and south with large ethnic Russian populations.
The OSCE's current chairman, Swiss President Didier Burkhalter, has proposed a "roadmap" aimed at defusing the crisis, visiting the Kremlin last week for talks with Russian President Vladimir Putin.
The OSCE has said that Putin had assured Burkhalter in a phone call that he was "supportive" of the plan, which focuses on stopping violence as well as disarmament, national dialogue and elections.
Russia has said it expects pro-Moscow rebels to comply with the roadmap - provided that Kiev authorities stop a military operation in the east and pull back troops.
"We call on the OSCE to secure agreement from Kiev authorities to fully cooperate on these issues," the Russian foreign ministry said.
"The implementation of... the 'roadmap' would create conditions for the start of a broad national dialogue aimed at reconciliation and an all-encompassing constitutional reform designed to prevent the country from further sliding toward catastrophe."