The Organisation for Security and Cooperation in Europe said Wednesday its website had been hacked.
The OSCE said it did not know who was behind the attack, which consisted of thousands of simultaneous requests to load the website being launched from different locations.
OSCE media freedom representative Dunja Mijatovic called the attack "unacceptable", and said there were "forces set in motion trying to limit freedom of expression."
The organisation has emerged as a key player in attempts to resolve the crisis in Ukraine, where it has been the target of a number of kidnappings by separatists. It currently has nearly 300 civilian observers deployed in the country.
It was formed in the 1970s as a forum for East-West dialogue during the Cold War. It now has 57 member states, including the United States, Canada, Russia, Mongolia and others throughout Europe.
Explore further: Internet access a fundamental human right: OSCE