MOSCOW (Reuters) - Russian Foreign Minister Sergei Lavrov told United States Secretary of State John Kerry on Saturday that Ukraine must stop its military operation in southeastern regions of the country as part of efforts to defuse the crisis, the Russian Foreign Ministry said.

In a telephone conversation with Mr Kerry, Mr Lavrov also urged the United States to use its influence to secure the release of what the Russian ministry called leaders of the "protest movement" in south-eastern Ukraine - a reference to pro-Russian separatists.

The ministry said Mr Kerry and Mr Lavrov discussed efforts to resolve the situation involving detained OSCE military observers.

It said "public structures" controlling parts of south-eastern Ukraine had not been properly informed of the observers' plans to travel there.