"Russian and Ukrainian delegations are working on the entire spectrum of contentious issues," Ukrainian Presidential Advisor Mykhailo Podolyak said in a tweet posted on Tuesday, with a picture of Ukrainian representative David Arakhamia seated with the head of Russia's negotiation team Vladimir Medinsky.

The heads of the Russian and Ukrainian delegations have begun one-to-one talks on Tuesday in Istanbul, with the meeting among all negotiators to follow, a CMG reporter has learned.