The work is taking place near the former village of Puźniki, now known as Puzhnyky.
Professor Andrzej Ossowski of Pomeranian Medical University, who leads the Polish side, told the Polish news agency PAP that the team is searching for the remains of around 90 people.
He expects the search, focused on an unexamined part of the village cemetery, to take about a week.
Remains of 43 victims were already found and reburied in September 2025.
Ossowski said witness accounts point to roughly 90 victims in total, meaning a second grave is likely still out there.
The search is being conducted by Polish and Ukrainian forensic specialists.
Representatives of the Freedom and Democracy Foundation, which has long advocated the search and exhumation of victims at the site, are also present, along with government officials and diplomats from both countries.
On the night of 12-13 February 1945, fighters from the Ukrainian Insurgent Army (the UPA), led by Petro Chamchuk, killed between 50 and 120 Poles in Puźniki, according to varying sources.
The excavation is part of a wider resumption of exhumation work in Ukraine since Kyiv lifted a ban on searching for Polish war victims' remains in November 2024.
That ban had been in place since 2017, after a UPA monument was dismantled in south-eastern Poland.
Other recent searches include sites at Ostrówki, Wola Ostrowiecka and a former Polish army burial ground in Lviv, while a search in Huta Pieniacka uncovered what could be the remains of about 100 people, with exhumation expected next year.
The killings in Puźniki form part of a long-running dispute over wartime history between Poland and Ukraine.
Warsaw says about 100,000 Polish civilians were killed by OUN (the Organisation of Ukrainian Nationalists ) and UPA forces between 1943 and 1945 and regards the massacres as genocide.
(ał)
Source: PAP