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

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

Vyacheslav Ponomarev, the self-proclaimed "people's mayor" of Slovyansk, described the detained observers as "captives" and said 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 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 tries 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 Friday night 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.

Condemning Russia's earlier annexation of Ukraine's Crimea Peninsula, 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."

The European Union also plans 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 consisted 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, said 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 late Friday to press for the release of the observers. In a statement released Saturday, Russia's Foreign Ministry said it was taking "all measures to resolve the situation," but blamed the authorities in Kiev for failing to secure the safety of the team.

"The security of the inspectors is wholly entrusted to the host party," the statement said. "Hence it would be logical to expect the current authorities in Kiev to resolve preliminary questions of the location, actions, and safety of the instructors."

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

Tymoshenko, who is running in the May 25 presidential election, said while only a minority of Ukrainians had supported NATO membership, Russia's aggressive actions in the country's east had forced a "fundamental change" in public thinking.

Her tough talk underscores the 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, Yatsenyuk 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."

In a briefing with reporters, he lashed out at Moscow, saying Russian military aircraft violated Ukrainian air space late on Friday.

"The only reason is to provoke Ukraine to strike a missile and to accuse Ukraine of waging a war to Russia," he said, and asked Russia "not to provoke and not to support Russian-led terrorists... in eastern and southern Ukraine. We ask Russia to leave us alone."

The Russian Defense Ministry denied claims, first raised by the U.S. on Friday, that its aircraft had crossed the border with Ukraine, a spokesman told state news agencies on Saturday.

The streets of Slovyansk were relatively calm Saturday. Hundreds of mourners, including Ponomarev, went to a local church to pay respects to a pro-Russian insurgent apparently killed during a clash with Ukrainian government troops earlier in the week.

Ukraine's acting president this week ordered security forces to resume operations in the country's east after the bodies of two people allegedly abducted by pro-Russia insurgents were found and a military aircraft was reported to have been hit by gunfire.

That came despite an international agreement calling for all sides in Ukraine to refrain from violence and for demonstrators to vacate public buildings. It didn't specifically prohibit security operations, but Ukraine suspended an earlier so-called "anti-terrorist operation" after the accord.

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Associated Press writers Laura Mills in Moscow, John-Thor Dahlburg in Brussels, Frances D'Emilio in Vatican City, Pietro De Cristofaro in Rome, and Frank Jordans in Berlin contributed to this report.