The head of the United Nations will travel to Russia next week to meet President Vladimir Putin.
The body says Secretary-General António Guterres will visit Moscow on Tuesday.
He'll have a working meeting and lunch with Russian Foreign Minister Sergey Lavrov, and will be received by the president.
It comes as thousands of people could be buried in mass graves detected near the besieged Ukrainian city of Mariupol, according to the city’s deputy mayor.
Sergei Orlov earlier said that about 21,000 people had been killed in the city since the siege began.
He said bodies were lying on the streets for many weeks after the invasion began - however, once the massacre at Bucha hit international headlines, Russian troops began hiding the bodies.
He said the massacre in Mariupol is about 10-times bigger than that carried out in Bucha.
Mr Orlov said Mariupol "does not exist anymore as a city" and noted that 90% of it is damaged and 45% of it is destroyed.
He said around 900 condominiums have been "totally destroyed" alongside around 21,000 private homes.
"As you can understand from that level of destruction, we have a lot of victims," he said.
"In our estimation, the total number of killed people is about 21,000 in our city.
"Several weeks ago, 10 days ago, one month ago, there were a lot of dead bodies lying on the street and at some point, they started to disappear."