Air raid sirens sounded across Ukraine before dawn on Tuesday as Ukrainian and Russian negotiators prepared to meet in Turkey for face-to-face talks, with Kyiv seeking a ceasefire without compromising on territory or sovereignty.

Ukraine and the United States hold little hope of a breakthrough at the meeting later on Tuesday, the first direct talks between the two sides in more than two weeks, even though Russia's invasion appeared to have stalled on several fronts.

More than a month into the war, the biggest attack on a European nation since World War Two, more than 3.8 million people have fled abroad, thousands have been killed and injured, and Russia's economy has been pummelled by sanctions.

In the besieged southern port city of Mariupol nearly 5,000 people have been killed, including about 210 children, according to figures from the mayor. Reuters was unable to immediately verify the numbers.

Survivors have told harrowing tales of people dying from lack of medical treatment, bodies being buried wherever space could be found, and women giving birth in basements.

Ukrainian Foreign Minister Dmytro Kuleba said of the talks in Turkey: "We are not trading people, land or sovereignty."

"The minimum programme will be humanitarian questions, and the maximum programme is reaching an agreement on a ceasefire," he said on national television.

A senior US State Department official said Russian President Vladimir Putin did not appear ready to make compromises to end the war.

Ukrainian interior ministry adviser Vadym Denysenko said that he doubted there would be any breakthrough.

Kremlin spokesperson Dmitry Peskov said talks so far had not yielded any substantial progress but it was important they continued in person. He declined to give more information.

Energy supplies

In an address on Monday night, Ukrainian President Volodymyr Zelenskiy repeated calls for the West to go further in punishing Moscow for its invasion.

"We, people who are alive, have to wait. Doesn't everything the Russia military has done to date warrant an oil embargo?"

While Western countries have imposed a series of hard-hitting sanctions upon Moscow, Europe is heavily reliant on energy imports from Russia and has been so far reluctant to act to block them.

US and German government officials are due to meet in Berlin this week with energy industry executives to discuss ways to boost alternative supplies for Germany.

The Kremlin has demanded that what it deems "unfriendly" countries pay for Russian gas in roubles, not euros, but is trying to decide a mechanism by which that could work.

Japanese Chief Cabinet Secretary Hirokazu Matsuno said on Tuesday that Japanese companies would be requested to refuse if Russia asked for rouble payments.

Tokyo also banned the export of luxury goods to Russia, effective April 5, in its latest response to the war. Prohibited items included luxury cars, motorcycles and fashion items.

Trapped

In besieged Ukrainian cities where conditions are desperate, the threat of Russian attacks has blocked exit routes for civilians, two Ukrainian officials said.

In Mariupol, the mayor said about 160,000 people were trapped.

"There is no food for the children, especially the infants. They delivered babies in basements because women had nowhere to go to give birth, all the maternity hospitals were destroyed," a grocery worker from Mariupol who gave her name only as Nataliia told Reuters after reaching nearby Zaporizhzhia.

As the humanitarian toll continues to rise, the United Nations said it had been able to bring food and medical supplies into Kharkiv, Ukraine's second-biggest city and one of its hardest hit.

"The enemy continues to vilely carry out missile and bomb strikes in an attempt to completely destroy the infrastructure and residential areas of Ukrainian cities," the Ukraine military's general staff said in a Monday briefing.

"(They) focus on fuel storage facilities in order to complicate logistics and create the conditions for a humanitarian crisis."

'Changing their goals'

Russia calls its actions in Ukraine a "special military operation" to disarm and "denazify" its neighbour. Ukraine and the West say Putin launched an unprovoked war of aggression.

Tuesday's talks will be the first in person since an acrimonious meeting between foreign ministers on March 10, a sign of shifts behind the scenes as Russia loses battlefield momentum.

"We have destroyed the myth of the invincible Russian army. We are resisting against the aggression of one of the strongest armies in the world and have succeeded in making them change their goals," Kyiv Mayor Vitali Klitschko said.

He said 100 people had been killed in the capital, including four children, and 82 multi-storey buildings had been destroyed. It was not possible to verify the figures.

Russia's military signalled last week it would concentrate on expanding separatist-held territory in eastern Ukraine, but Kyiv said Russian troops continued to try to surround the capital.

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

Since then, they have held talks via video link and publicly discussed a formula under which Ukraine might accept some kind of neutral status.

But neither side has budged over Russia's territorial demands, including Crimea, which Moscow seized and annexed in 2014, and eastern territories known as the Donbas, which Moscow demands Kyiv cede to pro-Russian separatists.