U.S. Secretary of State John Kerry and his Russian counterpart, Sergei Lavrov, have spoken by telephone about the escalating crisis in Ukraine.
Kerry hailed the release on May 3 of seven OSCE observers who were being held by pro-Russian separatists in eastern Ukraine but said the key is for Russia to withdraw its support for pro-Russian separatists who have seized government buildings across eastern Ukraine.
For his part, Lavrov told Kerry that Washington should use its influence to persuade the Kyiv "regime" to stop its military offensive against the rebels.
He called for an increased role for OSCE mediators in an effort to implement the Geneva agreement and to foster a national dialogue, primarily on constitutional reform.
Russia's Foreign Ministry said on May 3 that Lavrov and Swiss President Didier Burkhalter had "confirmed the need for the complete fulfillment" of the Geneva accord.
The Kerry-Lavrov call came as Ukraine's Interior Ministry said its forces had retaken the headquarters of the security services in the city of Kramatorsk as an offensive against pro-Russian separatists in the east continues.
The ministry said earlier that its forces had also regained control of the city's airport and a television tower and controlled all checkpoints around the city.
Kramatorsk is south of the rebel stronghold of Slovyansk, which the Interior Ministry said has also now been surrounded by its troops.
Ukrainian Interior Minister Arsen Avakov said military operations against pro-Russian separatists in the east had resumed at dawn on May 3.
The Ukrainian military offensive continued as Moscow blamed the government in Kyiv for the large loss of life in Odesa on May 2.
The Interior Ministry said 42 people -- most of them believed to be pro-Russian separatists -- died in the southern port city. Most of the dead perished in a fire in a trade-union building that is believed to have been started by pro-Kyiv supporters throwing petrol bombs.
On May 3, Ukraine's State Security Service (SBU) blamed "foreign interference" for the clashes in Odesa. An SBU spokeswoman alleged that illegal military groups from Moldova's breakaway region of Transdniester and Russian groups had worked together to foment the unrest.
WATCH: Slovyansk Rebels Release OSCE Observers
Acting Ukrainian President Oleksandr Turchynov has declared two days of mourning for those killed.
Dmitry Peskov, a spokesman for Russian President Vladimir Putin, told reporters in Moscow on May 3 that "the authorities in Kyiv are not only directly responsible, they are direct accomplices in these criminal actions. Their hands are full of blood."
Peskov also said the Kremlin "no longer has influence" over Russian speakers in Ukraine's east but promised to "do whatever is possible and wherever it is possible" to de-escalate the situation.
Asked how Russia would respond to the escalating crisis, Peskov said, "I cannot answer this question. It's an absolutely new element for us."
Peskov also said Moscow had been receiving "thousands of calls" from eastern Ukraine requesting "active help."
Russia has had some 40,000 troops massed on Ukraine's border for the past two months.
With reporting by Reuters, AFP, AP, and UNIAN