KINSHASA, Congo  — U.S. Secretary of State John Kerry calls the release of seven European military observers Ukraine a positive step, but says more is needed to ease tensions.

Kerry, traveling in Africa, spoke by telephone with Russian Foreign Minister Sergey Lavrov on Saturday.

Kerry says 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.

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

Meanwhile, Lavrov said he had asked his Kerry to press Ukraine to halt its military operation in the east of the ex-Soviet country, his ministry said.

“The minister called on the United States to use all of its influence to force the Kiev regime, which it looks out after and which has declared a war against its own people, to immediately halt military operations in the southeast, pull back its troops and free protesters," the foreign ministry said.

Kerry is 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.