Russia has reportedly sent mercenaries from an elite private military contracting business to eastern Ukraine, as its invasion of its neighbour becomes bogged down.

British intelligence report say more than 1000 mercenaries from Russia’s government-aligned Wagner Group had been sent to the embattled corner of Ukraine known as the Donbas.

“They are expected to deploy more than 1000 mercenaries, including senior leaders of the organisation, to undertake combat operations,” British Air Vice-Marshall Mick Smeath said.

“Due to heavy losses and a largely stalled invasion, Russia has likely been forced to reprioritise Wagner personnel for Ukraine at the expense of operations in Africa and Syria.”

Russian President Vladimir Putin had previously appealed to foreign fighters and organisations such as Wagner to join his cause in Ukraine. The company has also been sanctioned by Western nations following Moscow’s attacks on Ukraine.

This week’s report of Wagner’s deployment came as other Western officials say the surprising strength of Ukraine’s resistance had rattled Mr Putin’s invasion plan.

It also came just hours before the first face-to-face peace talks between Ukraine and Russia in more than two weeks were due to get underway in Istanbul.

Moscow’s more than month-long invasion of its neighbour has already prompted more than 3.8 million people to flee abroad, left thousands dead or injured, and brought the isolation of Russia’s economy.

Nearly 5000 people have been killed, including about 210 children, in the port city of Mariupol amid heavy Russian bombardment, according to figures from the mayor.

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

A Ukrainian delegation arrived in Turkey on Monday ahead of negotiations planned for Tuesday (local time), TV footage showed.

“The minimum program will be humanitarian questions, and the maximum program is reaching an agreement on a ceasefire,” Ukraine’s foreign minister Dmytro Kuleba said on national television.

“We are not trading people, land or sovereignty.”

Ukrainian negotiators arrive at Ataturk Airport in Istanbul, ahead of peace talks with Russia. Photo: Getty

Mr Putin, however, did not appear ready to make compromises to end the war, a senior US State Department official told Reuters on Monday on condition of anonymity.

And Ukrainian interior ministry adviser Vadym Denysenko said he doubted “there will be any breakthrough on the main issues”.

Kremlin spokesman Dmitry Peskov said talks so far had yielded no substantial breakthroughs, although it was important they continued in person.

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

In Mariupol, the mayor said 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.

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 mayor of Irpin, near the capital Kyiv, said Ukrainian forces were back in full control.

A US defence official said the Ukrainians had recaptured the eastern town of Trostyanets, south of Sumy, and were continuing to try to take back ground.

Russia’s defence ministry said its troops had destroyed ammunition depots in the Zhytomyr region west of Kyiv and had hit 41 Ukrainian military sites in the past 24 hours.

Reuters could not immediately verify any of the reports.

Tuesday’s talks will be the first in person since an acrimonious meeting between foreign ministers on March 10. They are considered a sign of shifts behind the scenes as Russia’s invasion has stalled and sanctions have hit home.

“We have destroyed the myth of the invincible Russian army,” Kyiv mayor Vitali Klitschko said.

“We are resisting against the aggression of one of the strongest armies in the world and have succeeded in making them change their goals.”

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

Russia’s military signalled last week it would concentrate on expanding territory held by separatists in eastern Ukraine. However, Ukraine said it saw no sign Russia had given up a plan 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 Mr Lavrov said a halt to fighting was not even on the agenda.

Since then, talks have been held via video link and there have been public discussions of a formula under which Ukraine might accept some kind of formal neutral status.

Biden sticks with Putin comments

US President Joe Biden says his remark in Warsaw that Mr Putin should be removed from power reflects his own moral outrage, not an administration policy shift.

“I wasn’t then nor am I now articulating a policy change,” he said at the White House, noting that prior to the remark, made in a speech on Saturday, he had visited families displaced by Russia’s invasion of Ukraine.

“I was expressing moral outrage that I felt, and I make no apologies.”

At the end of the speech in the Polish capital, Mr Biden added an unscripted line, saying Mr Putin “cannot remain in power”.

Administration officials rushed to clarify afterwards that the White House was not advocating for regime change in Russia.

Mr Biden added on Monday he was “not walking anything back” by clarifying the remark.

“I don’t care what he thinks. He’s going to do what he’s going to do,” he said.

But Mr Biden again suggested Mr Putin should not lead Russia.

If Mr Putin “continues on the course that he’s on, he’s going to become a pariah worldwide and who knows what he becomes at home in terms of support”, Mr Biden said.

-with AAP