SLOVYANSK, Ukraine - As Western governments vowed to impose more sanctions against Russia and its supporters in Ukraine, a group of foreign military observers remained in captivity Saturday, accused of being NATO spies by a pro-Russian insurgency.

The German-led team of eight was traveling under the auspices of the Organization of Security and Cooperation in Europe when members were detained Friday.

Vyacheslav Ponomarev, the self-proclaimed "people's mayor" of Slovyansk, described the detained observers as "captives" and said that they were officers from NATO member states.

"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," Ponomarev said, adding that they could be released in exchange for jailed pro-Russian activists.

Outside Slovyansk, a city about 90 miles west of Russia, Ukraine government forces continued operations to form a security cordon as it attempts to quell unrest threatening to derail the May 25 presidential election.

The U.S. and other nations in the Group of Seven said in a joint statement released Friday night by the White House that they plan to impose additional economic sanctions on Russia in response to its actions in Ukraine. The West has accused Russia of using covert forces to encourage unrest in Ukraine and says Moscow has done nothing to pressure pro-Russian militias to free police stations and government buildings in at least 10 cities across the region.

The European Union is also planning more sanctions and ambassadors from the bloc's 28 member nations will meet Monday in Brussels to add to the list of Russian officials and pro-Russian leaders in Ukraine that have been sanctioned with asset freezes and a travel ban.

The foreign military observer team detained by pro-Russian forces was made up of three German soldiers, a German translator and one soldier each from Czech Republic, Poland, Sweden and Denmark. Germany's Defense Ministry said the team 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.

Former Ukrainian Prime Minister Yulia Tymoshenko said in an interview Saturday that she welcomed further sanctions against Russia and called for NATO membership for Ukraine to protect itself from Russian aggression.

Her words underscored the increasingly tense relationship between Russia and Ukraine in recent weeks - acting Prime Minister Arseniy Yatsenyuk warned Friday that Russia "is keen to start World War III."

On Saturday, he traveled to Rome to meet with Pope Francis and Italian Premier Matteo Renzi. Francis gave Yatsenyuk a fountain pen, telling him, "I hope that you write 'peace' with this pen." Yatsenyuk replied: "I hope so, too."