A leader for pro-Russian separatists in eastern Ukraine said on Saturday that a group of foreign military observers who have been detained as suspected “NATO spies” could be released in exchange for jailed pro-Russian activists.

“They are officers from NATO member states,” said Vyacheslav Ponomaryev, self-proclaimed mayor of the eastern city of Slovyansk. “As we found maps on them containing information about the location of our checkpoints, we get the impression that they are officers carrying out a certain spying mission.”

The German-led, eight-member team was travelling under the auspices of the Organization of Security and Cooperation in Europe when they were detained in Slovyansk.

Germany’s Defense Ministry said it had had lost contact with the team, which it said also included five Ukrainians. Tim Guldimann, the OSCE’s special envoy for Ukraine, told German public radio WDR on Saturday that “efforts are being made to solve this issue.” He declined to elaborate.

German Foreign Minister Frank-Walter Steinmeier called Russian Foreign Minister Sergey Lavrov on late Friday to press for the release of the observers. A Russian embassy official was also called into the German Foreign Ministry to receive the same message.

Russia’s envoy to the OSCE, Andrei Kelin, told Russian state news agency RIA Novosti in Vienna that Russia was taking “all possible steps” to free the military observers.

With tensions rising in Ukraine, the United States and its partners in the Group of Seven said on Friday night by the White House that they were prepared to impose additional economic sanctions on Russia. The White House said an announcement could come as early as Monday.

The West has accused Russia of using covert forces to encourage unrest in Ukraine, and says Moscow has done nothing to pressure separatist militias to free police stations and government buildings in at least 10 cities across the region.

Condemning Russia’s annexation of Crimea, the G-7 said- “We will now follow through on the full legal and practical consequences of this illegal annexation, including but not limited to the economic, trade and financial areas.”

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