DONETSK, Ukraine — International negotiators rushed to eastern Ukraine on Saturday to seek the release of European military monitors who were captured Friday and promptly branded “spies” by the pro-Russia militia that seized them.

The detention of the monitors raised the stakes in an already fraught drama pitting the Ukrainian government against bands of separatists who have overtaken city halls across the country’s eastern half.

Although the standoff in Ukraine has for months been a proxy fight between Russia and the West, the imprisonment in a makeshift separatist jail of military officers from NATO countries threatens to draw the West more directly into the conflict.

Russia said Saturday that it would do all it could to win the release of the detained men, who include eight military monitors from Germany, the Czech Republic, Poland, Denmark and Sweden as well as five Ukrainian military escorts. But as of Saturday night, leaders of the self-proclaimed Donetsk People’s Republic in the city of Slovyansk remained adamant that they had no intention of freeing the officers, accusing them of espionage. Ukrainian officials said they feared that the men would be used as human shields.

The standoff raised fresh questions about the ability of any government – Ukrainian or Russian – to control events in a region where security is perilous, and where shadowy militias hold growing sway.

The Organization for Security and Co-operation in Europe, which is monitoring events in Ukraine and trying to broker local peace deals, said it would keep its monitors out of Slovyansk until further notice, and that it was carefully watching conditions in other cities.

“It’s a very fluid security situation in a lot of these areas,” said Michael Bociurkiw, spokesman for the OSCE monitoring mission.

Bociurkiw said OSCE monitors visited Slovyansk last week, and met with the pro-Russian activists who took over the city’s government buildings. “It was tense, but there was an understanding achieved,” he said. The detention of the monitors, he said, was “entirely unexpected.”