The Russian and Ukrainians delegations have now arrived at the Dolmabahce Palace on the banks of the Bosphorus in Istanbul.
Turkish President Recep Tayyip Erdogan addressed the delegations at the beginning of talks.
These are the first face-to-face talks in over a fortnight but hopes of any breakthrough are low.
Erdogan spoke of his “deep sorrow” that conflict has entered its fifth week, saying Turkey’s friendship with both countries gave it an obligation to mediate.
“It is in the hands of both sides to stop this tragedy,” he said.
Erdogan said both Ukraine’s Zelensky and Russia’s Putin are “valuable friends.”
He said progress at the talks would pave the way for the leaders to meet – and that Turkey could host that too.
Turkey is a Nato member but retains strong ties with Russia. Erdogan has resisted joining the West’s sanctions against Moscow.