Pro-Russian rebels release international monitor, West prepares sanctions

On the same day, President Barack Obama said the U.S. and Europe must join forces to impose sanctions on Russia to prevent it from further destabilizing Ukraine.

After a joint statement made by the Group of Seven on Friday calling for a new round of sanctions, the U.S. named new people and firms close to Russian President Vladimir Putin to be hit by punitive measures. The European Union is expected to follow suit, but there is no consensus yet on wider economic sanctions.

Speaking during a visit to Malaysia on Sunday, Obama said any decision on whether to impose sanctions on sectors of the Russian economy at a later time would depend on whether the U.S. and its allies could find a unified position on how to proceed.

"We're going to be in a stronger position to deter Mr. Putin when he sees that the world is unified and the United States and Europe is unified rather than this is just a U.S.-Russian conflict," Obama told reporters.

Obama said Russia had not "lifted a finger" to compel pro-Russian armed groups in Ukraine to comply with an international agreement reached in Geneva to defuse the crisis.

"In fact, there's strong evidence that they've been encouraging the activities in eastern and southern Ukraine," he said.

Since Ukrainians toppled their pro-Russian president in February, Russia has responded by annexing Ukraine's Crimean peninsula and massing tens of thousands of troops on the country's eastern border. 

The Western-backed government in Kyiv accuses Russia of planning to invade mainly Russian-speaking eastern Ukraine and of preparing the ground by training and supporting pro-Russian armed rebels who have seized about a dozen public buildings in the region.

Moscow denies interfering. It says Ukraine's east is rising up in a spontaneous protest against what it calls an illegitimate government in Kyiv, which is mounting a "criminal" operation to suppress dissent.

Washington has largely been more intent on sanctions than its counterparts across the Atlantic, with many European countries worried that such measures could affect their commercial ties with Moscow. The EU imports about a quarter of its natural gas from Russia.

Meanwhile, Ponomaryov said Sunday that his men captured three officers with Ukraine's state security service overnight who had been mounting an operation against pro-Russian armed groups in the nearby town of Horlivka.

The Russian television station Rossiya 24 showed footage it said was of a colonel, a major and a captain. They were shown seated, with their hands behind their backs, blindfolded and wearing no trousers. At least two had bruises on their faces.

Ukraine's State Security Service said the three are part of a unit that went to Horlivka to arrest a suspect in the murder of Volodymyr Rybak, a pro-Kyiv councilor whose body was found last week in a river near Slovyansk.

Al Jazeera and wire services