The Kremlin press service on May 1 reported that German Chancellor Angela Merkel has asked Russian President Vladimir Putin to help free a team of OSCE observers that had been captured in eastern Ukraine.
The Kremlin said following phone talks between the two leaders that "Angela Merkel made a request to help facilitate the release of military observers from a number of European countries including Germany."
It added, "Both sides noted the importance of utilizing the mediating potential of the OSCE across Ukraine to the maximum degree."
Pro-Russian separatists who have been holding the group of OSCE observers since April 25 have variously said the captives are "prisoners of war" and "NATO spies."
One of the eight original captives was released for medical reasons.
Earlier on May 1, Russia's Foreign Minister Sergei Lavrov called for OSCE-sponsored talks between Kyiv and pro-Russian separatists in southeastern Ukraine.
"Russia believes that such a dialogue could be arranged between the authorities in Kyiv and their opponents in other regions of the country under the aegis of the Organization for Security and Cooperation in Europe (OSCE)," Lavrov told Rossiya-24 TV during a visit to Peru.
Ukraine accuses Russia of being behind the actions of pro-Russian separatists who have taken over more than a dozen towns and cities in the east of the country.
Meanwhile, Ukrainian authorities on May 1 detained Russia's military attache to Kyiv on suspicion of spying and ordered the diplomat to leave the country.
In a statement, the Ukrainian Foreign Ministry said the unnamed diplomat was detained on April 30 while undertaking "intelligence activities."
The statement mentioned no deadline for the diplomat's departure. There was no immediate response from Moscow, which like Kyiv is observing the May 1 holiday.
The move came a day after Ukraine's acting president said on April 30 that Ukraine's army is on "full combat alert" as pro-Russian separatists continued to seize administrative headquarters in the east of the country.
Oleksandr Turchynov told a ministerial meeting in Kyiv that "the threat of Russia starting a war against mainland Ukraine is real."
Turchynov also was critical of the country's "helpless" security forces, saying they were "unable to carry out their duties of protecting citizens."
Separatists have occupied government buildings and police headquarters in about a dozen towns and cities in eastern Ukraine in recent weeks.
After seizing key buildings in the capital of the easternmost province, Luhansk, on April 29, pro-Russian activists took control on April 30 in the nearby towns of Horlivka and Alchevsk.
In Donetsk, the biggest city to fall under the control of Russian-speaking separatists, a "People's Republic of Donetsk" was declared and a referendum on secession was announced for May 11.
Ukraine's presidential election is scheduled two weeks later, on May 25.
Based on reporting by Reuters and Interfax-Ukraine