Pro-Russian militants in camouflage fatigues and black balaclavas paraded captive European military observers before the media Sunday, hours after three Ukrainian security guards were shown on Russian TV bloodied, blindfolded and stripped of their trousers and shoes, their arms bound with packing tape.

The provocative displays came as the increasingly ruthless pro-Russian insurgency in the east turns to hostage-taking as an ominous new tactic.

Germany’s foreign minister condemned the appearance as “revolting” and a violation of the men’s dignity. Four members of the team are German. One of the observers, a Swedish officer, was released later in the day.

Dozens of people are being held hostage, including journalists and pro-Ukraine activists, in makeshift jails in Slovyansk in the heart of the separatists’ territory, as the pro-Russian insurgents strengthen their control in the east in defiance of the interim government in Kiev and its Western supporters.

Col. Axel Schneider from Germany, who spoke for the group of military observers detained Friday, stressed that they were on a diplomatic mission under the auspices of the Organization of Security and Cooperation in Europe and weren’t spying for NATO, as the insurgents claim.

Schneider said additional proof of this was the participation of the officer from Sweden, which is not a member of NATO.

The observers appeared nervous as they were escorted by armed men into the Slovyansk city hall for the news conference and then led away.

Referring to himself and his team as “guests” under the “protection” of the city’s self-proclaimed mayor, Schneider said they were being treated as well as possible under the circumstances.

“The mayor of this city granted us his protection and he regarded us as his guests,” Schneider told journalists. “I can tell you that the word of the mayor is a word of honor. We have not been touched.”

Schneider, who was speaking before the Swede was freed, said he had no information about when they would be released and that this was a matter for diplomats of their countries. The group also includes officers from Poland, Denmark and the Czech Republic.

The Swedish officer, Maj. Thomas Johansson, got into a car with OSCE representatives outside city hall and drove off with them. He was freed “on humanitarian grounds as he has a mild form of diabetes,” said Stella Khorosheva, a spokeswoman for the city’s mayor, Vyacheslav Ponomarev.

German Foreign Minister Frank-Walter Steinmeier condemned the display of the prisoners.

“Today’s public display of the OSCE observers and the Ukrainian security forces as prisoners is revolting and violates (their) dignity in a blatant fashion,” he said in a statement. “This is a breach of all the rules and standards.”

The German colonel said he understood that the Slovyansk mayor could use the observers as a bargaining chip.

Ponomarev said Saturday that the European observers could be released in exchange for jailed pro-Russia activists.

The mayor has refused to specify how many Ukrainian journalists and activists his forces have detained, but he suggested it was several dozen.

On Sunday, the insurgents captured three Ukrainian security service officers, who were shown to Russian journalists in the security service headquarters in Slovyansk. Stripped of their trousers and shoes, they sat with heads bowed.

Igor Strelkov, who has been identified as the commander of the armed insurgents, said the three Ukrainian officers were on a mission to seize leaders of the pro-Russia force when they were captured.