Ukraine said yesterday it was pausing evacuations of civilians from war-scarred regions of the country because intelligence reports suggested invading Russian troops were planning attacks on humanitarian routes.

"Our intelligence has reported possible provocations by the occupiers on routes of humanitarian corridors. Therefore, for reasons of public safety, we are not opening humanitarian corridors today," Deputy Prime Minister Iryna Vereshchuk said in a statement on social media.

Ukraine also warned that the humanitarian crisis in the pulverized city of Mariupol was now "catastrophic", while signalling grounds for compromise ahead of new face-to-face peace talks with Russia in Turkey. 

On the ground, there was no sign of respite for civilians in besieged cities, especially in Mariupol, whose mayor said 160,000 people were still trapped inside.

In the southern town of Mykolaiv, under heavy assault for weeks, the bombardments appeared to be easing.

One person was killed and another wounded by overnight Russian bombardment in the Lugansk city of Rubizhne, according to regional Ukrainian officials.

A senior Turkish official said the Istanbul talks were set begin yesterday, but the Kremlin later said they were not likely to begin till today, 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.

In an interview with Russian journalists at the weekend, Ukrainian President Volodymyr Zelensky 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, meanwhile, said yesterday it was preparing to restrict entry into Russia for nationals of "unfriendly" countries, which include Britain, all EU states and the United States.

"A draft presidential decree is being developed on retaliatory visa measures in response to the 'unfriendly' actions of a number of foreign states," Lavrov said in televised remarks.