Kyiv's delegation at Istanbul talks plans to discuss with Russian negotiators a potential meeting between Ukrainian President Volodymyr Zelensky and Russian President Vladimir Putin, a diplomatic source said.

Before the talks, a three-way meeting between Russian, Ukrainian and Turkish delegations was to be held in the Turkish presidency's Dolmabahce Palace.

Meanwhile, US Secretary of State Marco Rubio wrapped up an hour-long meeting with his Turkish and Ukrainian counterparts.

The talks, also held at Dolmabahce Palace, brought together Turkish Foreign Minister Hakan Fidan, Ukrainian Foreign Minister Andriy Sybiga and several other top-level officials from the three countries.

The later talks between Russia, Ukraine and Turkey will be the two warring countries' first direct peace talks in more than three years.

Zelensky had travelled to Turkey but said he would not attend the talks, after Putin declined his calls for face-to-face negotiations.

Ukraine aims to secure an "unconditional ceasefire" at the talks on Friday, Zelensky's top aide said.

"Ukraine is ready for peace and a long-term and unconditional ceasefire," Andriy Yermak wrote on Telegram, adding: "The Ukrainian delegation is in Istanbul today to achieve an unconditional ceasefire – this is our priority."

Adding to the stakes, the European Union is working on a new package of sanctions against Russia to increase the pressure on Putin, EU Commission president Ursula von der Leyen said just ahead the start of the meeting.

"We will increase the pressure," von der Leyen said before a meeting with European leaders in Tirana, Albania.

"We are working on a new package of sanctions, that includes sanctions against Nordstream 1 and 2, also listing more vessels in the shadow fleet, lowering the oil price cap and finally more sanctions on the Russian financial sector." (Agencies)