U.S. Secretary of State John Kerry called the release of seven European military observers in Ukraine a positive step on Saturday, but said more is needed to ease tensions.

Seven OSCE military observers and five Ukrainian assistants were seized on April 25 in the city of Slovyansk, the epicenter of eastern Ukraine's unrest, as they traveled with an Organization for Security and Cooperation in Europe observer team. The insurgents said they possessed unspecified suspicious material and alleged they were spying for NATO.

Kerry said it's important that Russia withdraw support for separatists who have seized government buildings in about a dozen cities and towns in Ukraine.

Kerry is also raising the prospect of additional Western sanctions if there are indications of continued interference by Russia.

Russia denies allegations it's fomenting the unrest in eastern Ukraine. Russia's Foreign Ministry responded to the release of OSCE observers from European countries, stating that it shows the "bravery and humanism" of the defenders of Slaviansk,

The ministry said that the step was taken "in conditions, when a peaceful town is coming under a direct, unmotivated attack..."

"Without doubt this is evidence of the bravery and humanism of the defenders of the town. They showed in the first place concern about the security of foreign citizens."

Kerry is traveling in Africa and spoke by telephone with Russian Foreign Minister Sergey Lavrov (SEHR'-gay LAHV'-rahf) on Saturday.

Kerry said they discussed the violence in Odessa, where at least 42 people died in clashes between government supporters and opponents Friday, and that everything possible must be done to end it.

Lavrov told Kerry the U.S. should use its influence to make Ukraine's government immediately stop military operations in south-east Ukraine, the Russian foreign ministry said in a statement on Saturday.

Lavrov also said that it was important that the mediating role of the OSCE was increased to secure Kiev's fulfillment of the Geneva declaration on de-escalating tensions in Ukraine. "Chances of this still exist," the statement said, as long as all Ukrainian regions are represented in a national dialogue on constitutional reform, and "terrorists" from the Right Sector group are curbed.

Meanwhile, Ukrainian forces retook the security service headquarters in the eastern town of Kramatorsk, south of rebel stronghold Slaviansk, the country's Interior Ministry said on Saturday.

"Now it is under the control of the National Guard," the ministry said in a statement. Local television showed pictures of armored personnel carriers moving through the town.