The localised truce in the devastated southern city of Oleshky began on Tuesday and is due to last through Sunday as a humanitarian operation races to get urgently needed food and medicine to about 2000 remaining residents as well as allow those who want to leave a path out.
If the ceasefire holds, it will be the first time since Russia's full-scale invasion of its neighbour more than four and a half years ago that the two sides have observed a significant truce.
Previous attempts to stop the fighting have either failed to take hold or quickly unravelled amid deep mistrust.
A representative from the United States will join civilian officials from Russia and Ukraine when they go into Oleshky, Ukraine's lead negotiator for the aid mission said.
"As soon as the road (into Oleshky) is de-mined and the humanitarian cargo is delivered there, representatives of three countries - Ukraine, the United States and the Russian Federation - will be present on the spot," Ukrainian ombudsman Dmytro Lubinets said.
Residents who want to leave will be free to choose between Ukraine and Russia as their destination, he said in an interview with Ukraine's private 1+1 television.
Authorities will try to ensure that aid will keep flowing into Oleshky for anyone who wants to remain, he said.
Negotiators involved in the aid mission said doctors, medicines, food supplies and water would be sent to Oleshky before any exit corridors are opened.
Russian forces impose strict limits on who can enter Oleshky and there was no news on how the humanitarian operation was unfolding.
Lubinets and his Russian counterpart Yana Lantratova both said on Wednesday there had been no ceasefire violations in Oleshky for 30 hours.
The plight of the city in Ukraine's southern Kherson region caught the attention of US first lady Melania Trump and the White House said that she brokered the humanitarian mission.
The ceasefire coincided with US-led plans to bring Russian and Ukrainian officials back to the table next month for technical talks on how to move forward on ending the war, even though neither side shows signs of backing down from their key demands.
Talks between delegations from Ukraine, Russia and the US that are expected to take place in Abu Dhabi, United Arab Emirates, in November have not been scheduled yet because the US is awaiting developments in the Middle East, Ukrainian President ‌Volodymyr Zelenskiy told reporters.
Zelenskiy said during a visit to the Netherlands on Wednesday that he hopes a date can be scheduled for the talks, possibly between November 6-10.
Russian President Vladimir Putin's foreign affairs adviser Yuri Ushakov meanwhile said that European representatives would not be invited to the meeting.
Zelenskiy said that the leaders of Finland and Norway, who spoke with Putin by phone on Tuesday, told him that Russia is "serious" about discussing steps to de-escalate the war.
The Ukrainian president said these could include a pause in attacks targeting energy infrastructure and commercial sea traffic, further prisoner exchanges and a discussion of formats for possible summit-level talks.
