Russian negotiators flew to Turkey to hold peace talks with Ukraine on Wednesday, the Kremlin said, before what will be the first direct discussions between the warring sides in more than seven weeks.

Russia played down expectations of any breakthrough at the meeting, which Ukrainian President Volodymyr Zelenskiy said this week should focus in part on preparing a summit between himself and President Vladimir Putin.

"Naturally, no one expects an easy road. Naturally, this will be a very difficult conversation. The projects (of the two sides) are diametrically opposed," Kremlin spokesman Dmitry Peskov told reporters.

Previous talks in Istanbul on May 16 and June 2 led to the exchange of thousands of prisoners of war and the remains of dead soldiers. But those meetings lasted less than three hours in total and made no breakthrough towards a ceasefire or a settlement to end almost three and a half years of war.

US President Donald Trump last week threatened heavy new sanctions on Russia and countries that buy its exports unless a peace deal was reached within 50 days.

But three sources close to the Kremlin told Reuters that Putin, unfazed by Trump's ultimatum, would keep on fighting in Ukraine until the West engaged on his terms for peace, and that his territorial demands may widen as Russian forces advance.

On Wednesday, Russia said its forces had captured the settlement of Varachyne in Ukraine's Sumy region, where Putin has ordered his troops to create a buffer zone after Ukraine mounted a shock incursion into Russia last year and held onto a chunk of its territory for months. Reuters could not independently confirm the battlefield report.

In recent weeks, Russian forces have launched some of their heaviest air attacks of the war, focusing especially on the Ukrainian capital Kyiv.

Ukraine has hit back with attacks of its own, and last month inflicted serious damage on Russia's nuclear-capable strategic bomber fleet by smuggling drones close to air bases deep inside the country.

CONFLICTING DEMANDS

Zelenskiy said earlier this week that the agenda for talks was clear: the return of prisoners of war and of children abducted by Russia, and the preparation of a meeting between himself and Putin.

Putin turned down a previous challenge from Zelenskiy to meet him in person and has said he does not see him as a legitimate leader because Ukraine, which is under martial law, did not hold new elections when Zelenskiy's five-year mandate expired last year. Russia also denies abducting children.

The Kremlin said this week it was unrealistic to expect "miracles" from the talks.

At the last meeting on June 2, Russia handed Ukraine a memorandum setting out its key demands, including: full withdrawal of Ukrainian forces from four regions of the country that Russia has claimed as its own; limits on the size of Ukraine's military; enhanced rights for Russian-speakers in Ukraine; and acceptance by Kyiv of neutral status, outside NATO or any other alliance.

Ukraine sees those terms as tantamount to surrender, and Zelenskiy described the Russian stance as an ultimatum.

Ukraine wants an immediate ceasefire, reparations, international security guarantees and no restrictions on its military strength.