Pro-Russian separatists in eastern Ukraine have freed the European monitors seized last month. The team, held for more than a week in the flashpoint town of Slavyansk, was released Saturday.
The German-led monitoring team was acting under the authority of a four-party agreement directing the Vienna-based Organization for Security and Cooperation in Europe - OSCE - to monitor security and human rights in Ukraine's Russian-speaking east and south. The deal reached in Geneva was signed by Russia, Ukraine, the United States and the European Union.
In developments Friday in Ukraine, police in the port city of Odessa said at least 31 people died in a building fire that broke out during fighting between pro-Russian separatists and supporters of the central government in Kyiv. Most of the victims were apparently members of pro-Russian groups.
Odessa, a key Black Sea port, had largely escaped the violence gripping large parts of eastern Ukraine.
A government statement said the fire broke out in a trade union building. Russia's RIA Novosti news agency said about 50 others were injured, including 10 police officers.
Earlier Friday, Ukrainian government forces launched the first major assault on the rebel-held eastern city of Slavyansk.
Western media said Ukrainian forces seized rebel checkpoints on the outskirts of the city, but have not advanced toward its heavily fortified center. Separatists shot down two Ukrainian helicopters during the fighting, killing two crew members. A third occupant was reported captured.
Ukraine's acting President Oleksandr Turchynov said rebels had suffered heavy losses, including many killed and wounded.
In Moscow, a spokesman for Russian President Putin called the Ukrainian offensive "a criminal act." He also said it had "effectively destroyed the last hope" for implementing an April 17 international accord aimed at defusing the crisis. Mr. Putin on Thursday demanded that Ukraine withdraw all military personnel from the troubled region near the Russian border.
In Washington, President Barack Obama -- speaking alongside German Chancellor Angela Merkel -- said that if Russia's leadership continues to destabilize eastern Ukraine, the U.S. and European Union will move quickly to impose additional penalties, including both diplomatic and economic sanctions.
In New York, Western powers and Russia used an emergency session of the U.N. Security Council to accuse each other of failing to take appropriate steps to de-escalate the crisis. British Ambassador Mark Lyall Grant said "the scale of Russian hypocrisy is breathtaking."
For his part, Russian Ambassador Vitaly Churkin accused the West of applying double standards in the crisis by condoning the current Ukrainian offensive, after urging then-Ukrainian president Viktor Yanukovych not to deploy troops when his government in Kyiv was under siege earlier this year.
Pro-Russian gunmen control a number of key buildings in the key eastern Ukrainian industrial city of Donetsk and have declared a May 11 referendum on whether to secede from Ukraine and join Russia.