WASHINGTON, April 26 -- U.S. Secretary of State John Kerry on Saturday called for Russian help in securing the release of European monitors detained by pro-Russia militants in eastern Ukraine.

In a telephone conversation with his Russian counterpart Sergei Lavrov, the third this week, the top American envoy appealed for Moscow's support "without preconditions" for the efforts of the Organization for Security and Cooperation in Europe (OSCE) and the Ukrainian government to win the release of the OSCE military inspectors and their Ukrainian guides.

The inspectors were sent to Ukraine under an agreement reached in Vienna on March 21 among the OSCE members, with a view to easing tension and defusing a military standoff in the country. Some inspectors were detained on Friday by an anti-government group in the city of Slovyansk.

Kerry spoke of Kiev's efforts to pass a bill granting amnesty to pro-Russia militants, who were occupying government buildings and engaging in conflicts with the Ukrainian military in the country's east, and efforts to promote "broad and deep" constitutional reform across the country.

"He called on Russia to publicly support Ukraine's efforts rather than denigrate them," a senior State Department official said in a readout of the phone talks.

"He also expressed continued concern that Russia's provocative troop movements on Ukraine's border, its support for separatists and its inflammatory rhetoric are undermining stability, security and unity in Ukraine," the official said.

For his part, Lavrov urged Washington to use its influence to help to secure the release of leaders of anti-government protests detained in southeastern Ukraine, according to the Russian Foreign Ministry.

Washington and Moscow have been trading blame for the unrest in eastern Ukraine. Russian troops began military drills on the border on Thursday to respond to Kiev's "anti-terror" operation in the east to flush out pro-Russian militants occupying government buildings as well as to NATO's preparations for drills in Poland and the Baltic states.

The leaders of the Group of Seven industrialized nations said on Friday that they had agreed to "move swiftly" to impose more sanctions on Russia in response to its inaction in helping to ease growing tension in eastern Ukraine.

In their phone talks, Kerry and Lavrov also discussed the ongoing efforts to remove chemical weapons from Syria, as 92 percent have been removed out of the country mired in a three-year conflict.

Kerry also expressed "strong concern" over reports of a recent attack using a toxic chemical, likely chlorine, the State Department official said.