US Secretary of State John Kerry urged Russia to stop supporting Ukrainian separatists as he talked by phone with his Russian counterpart Sergei Lavrov on Saturday, a day after postponing the call.

   Hailing the release of OSCE observers by pro-Russian rebels in the eastern Ukrainian flashpoint town of Slavyansk, Kerry said: "It's a step. But there are many other steps that have to be taken in order to be able to de-escalate the situation."

   In his phone call with Lavrov, Kerry said he reiterated "that it is important for Russia to withdraw support for the separatists and to assist in removing people from the buildings and beginning to de-escalate the situation."

   He also warned that "if those supported by Russia continue to interfere with the election, regrettably there will have to be different sanctions including the possibility or the reality of sector sanctions," referring to the Ukrainian presidential poll set for May 25.

   "But foreign minister Lavrov and I did talk about how to proceed and perhaps how to find a way forward here. We both will be in touch."

   On Friday, Kerry had postponed a scheduled phone call with his Russian counterpart, according to a Moscow diplomatic source quoted by Russian news agencies. (AFP)