Ukraine and Russia were preparing on Monday for the first face-to-face peace talks in more than two weeks, with Kyiv insisting it would make no concessions on Ukraine's territorial integrity as battlefield momentum has shifted in its favour.

Ukrainian officials played down the chances of a major breakthrough at the talks, due to be held in Istanbul after Turkish President Tayyip Erdogan spoke to Russia's Vladimir Putin on Sunday.

But the fact that they were taking place in person at all - for the first time since an acrimonious meeting between foreign ministers on March 10 - was a sign of shifts behind the scenes as Russia's invasion has become bogged down.

On the ground, there was no sign of respite for civilians in besieged cities, especially the devastated port of Mariupol, whose mayor said 160,000 people were still trapped inside, and he accused Russia of making it impossible to evacuate them.

A senior Turkish official said the Istanbul talks would start on Monday, but the Kremlin later said they were not likely to begin till Tuesday, adding it was important they take place face to face despite scant progress in negotiations so far.

Mykhailo Podolyak, head of the Ukrainian delegation, told Reuters the start time depended on when the delegations could get there.

Ukrainian officials have repeatedly suggested in recent weeks that they believe Russia could now be more willing to compromise, as any hope Moscow may have held of imposing a new government on Kyiv slipped away in the face of stiff Ukrainian resistance and heavy Russian losses.

Russia's military signalled last week it was shifting focus to concentrate on expanding territory held by separatists in eastern Ukraine, a month after having committed the bulk of its huge invasion force to a failed assault on Kyiv.

When the sides last met in person, Ukraine accused Russian Foreign Minister Sergei Lavrov of ignoring its pleas to discuss a ceasefire, while Lavrov said a halt to fighting was not even on the agenda.

Since then, they have repeatedly met via video link, rather than face to face. Both sides have publicly discussed progress on a diplomatic formula under which Ukraine might accept some kind of formal neutral status. But neither has budged over Russia's territorial demands, including Crimea, which Moscow seized and annexed in 2014, and eastern territories known as the Donbass, which Moscow demands that Kyiv cede to separatists.

"I don't think there will be any breakthrough on the main issues," Ukrainian interior ministry adviser Vadym Denysenko said on Monday.

In an interview with Russian journalists at the weekend, Ukrainian President Volodymyr Zelenskiy mentioned some form of "compromise" involving Donbass, although he did not suggest this might involve ceding the territory. In his latest comments overnight he made clear that "territorial integrity" remained Kyiv's priority at the talks.



Russia calls its actions in Ukraine a "special military operation" to disarm and "denazify" its neighbour. Kyiv and the West consider this a pretext for an unprovoked invasion.

From the outset, Western countries said they believed Russia's true aim was to swiftly topple the Kyiv government, which Moscow failed to achieve in the face of strong Ukrainian resistance, later leading to huge Russian losses.

Last week, Ukrainian forces went on the offensive, pushing Russian troops back in areas around Kyiv, the northeast and the southwest. Russia has meanwhile kept up pressure in the southeast near separatist areas, including its devastating siege of the port of Mariupol, razed to the ground with tens of thousands of civilians trapped inside for weeks.

The city's mayor, Vadym Boichenko, who has escaped the city and was speaking from an undisclosed location, said 160,000 civilians were still trapped there, without heat and power. Twenty-six buses were waiting to evacuate them but Russian forces had not agreed to give them safe passage.

"The situation in the city remains difficult. People are beyond the line of humanitarian catastrophe," Boichenko said on national television. "We need to completely evacuate Mariupol."

Ukraine's Deputy Prime Minister Iryna Vereshchuk said there were no plans to open corridors to evacuate civilians from besieged cities on Monday, because of intelligence reports of possible Russian "provocations" along the routes.

Elsewhere, Russia's armoured columns are bogged down, with trouble resupplying and making little or no progress, despite pounding residential areas.

"As of today, the enemy is regrouping its forces, but they cannot advance anywhere in Ukraine," Ukrainian Deputy Defence Minister Hanna Malyar said on Monday.

Britain's defence ministry also said there had been no major change in Russia's positions in the past 24 hours, with most Russian gains near Mariupol and heavy fighting underway there.

Ukraine's General Staff said Kyiv defence forces were holding back Russian troops trying to break through from the northeast and northwest and take over key roads and settlements. In the south, Ukrainian forces were focused on defending the cities of Krivy Rih, Zaporizhzhia and Mykolayiv.

(Reporting by Reuters bureaus; Writing by Peter Graff; Editing by Gareth Jones)