Turkish Prime Minister Recep Tayyip Erdogan will host his Russian counterpart Vladimir Putin in Ankara Thursday.
On the eve of the visit, Mr. Putin's deputy chief of staff, Yuri Ushakov, said Turkey has agreed in principle to allow construction of Moscow's planned South Stream pipeline in Turkish waters.
The pipeline will carry Central Asian gas from Russia under the Black Sea to Europe, bypassing Ukraine.
The two leaders are expected to sign a deal Thursday.
Mr. Putin's aide said the agreement does not mention any gas marketing rights for Turkey.
Doubts about Russia's reliability as a gas supplier resurfaced in January when Russia cut supplies to Europe during a pricing dispute with Ukraine.
The Moscow-backed South Stream pipeline will be competing with the European Union-backed Nabucco pipeline. Turkey and four members of the EU signed an agreement last month to construct that pipeline.
Some information for this report was provided by AP and Reuters.