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-Russian militia that seized them.
The detention of the monitors instantly raised the stakes in an already fraught drama pitting the Ukrainian government against motley 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 a total of 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.
Vyacheslav Ponomarev, the self-appointed ‘‘people’s mayor’’ of Slovyansk, said the detained observers were found with maps suggesting they were carrying out a spying mission, and said they might be released in exchange for jailed pro-Russian activists, the Associated Press reported.
The standoff raised fresh questions about the ability of any government — whether 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 Cooperation 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 here. ‘‘We’re definitely taking more precautions.’’
Bociurkiw said that OSCE monitors had visited Slovyansk this past week, and they had met with the pro-Russian activists who took over the city’s government buildings this month. ‘‘It was tense, but there was an understanding achieved,’’ Bociurkiw said.
The detention of the monitors Friday, Bociurkiw said, was ‘‘entirely unexpected.’’
The detained men are military officers who also were here under OSCE auspices, but under a separate mission from the civilian observers.
OSCE dispatched a team to eastern Ukraine on Saturday and was leading negotiations aimed at securing the monitors’ release, according to officials from OSCE and from Germany, which led the mission.
German Foreign Minister Frank-Walter Steinmeier called Russian counterpart Sergey Lavrov to discuss the situation Saturday, according to a Russian Foreign Ministry statement. Steinmeier’s spokeswoman declined to discuss the call.
The standoff comes at a critical time in Western decision-making over how hard to punish Russia for its meddling in Ukraine. In a statement released Saturday morning, the Group of Seven announced that it would impose new sanctions against Moscow, and officials indicated they could come as early as Monday.
Germany, which has extensive economic ties to Russia, has led the call in the West for restraint in dealings with Moscow. But the detention of the officers, including four Germans, in Slovyansk could alter that.