Polish Foreign Minister Radoslaw Sikorski told reporters that Polish consular services are working to secure his release. He gave no further details on the case.
Polish media named the priest as Father Pawel Witek and said was seized yesterday by separatist rebels.
Polish and Ukrainian media said the priest is believed to be at the Ukrainian SBU state security service building, which is occupied by the separatists in Donetsk.
Poland has been one of the staunchest supporters of the new pro-Western government in Ukraine.
A four-member team of monitors from the Organization for Security and Cooperation in Europe went missing in the Donetsk area on May 26.
#ukraine's culture minister, MC of Maidan's stage during the protests: one more big rally then let's clear the square http://t.co/Re4qAFJ0xR
— Gulliver Cragg (@gullivercragg) May 28, 2014
Out of donetsk after three weeks, leaving through Dnipropetrovsk. Not all that far away but a different world - Ukrainian flags everywhere
— Shaun Walker (@shaunwalker7) May 28, 2014
Rebel truck hit by #Ukraine military helo gunfire & flipped on side has been brought to rebel roadblock near airport pic.twitter.com/y9n56jJTKg
— Christopher Miller (@ChristopherJM) May 28, 2014
For war-nerds; what on earth is this? pic.twitter.com/mJKhmgrjFI
— Olaf Koens (@obk) May 28, 2014
Right, the majority thinks it's a semi-automatic anti-tank rifle from 1941 (!) http://t.co/gaKZeio6CU
— Olaf Koens (@obk) May 28, 2014
Ukraine says 18-22k Russian troops remain parked on its borders as Russia informs Ukraine it's been asked to provide humanitarian aid
— bruce springnote (@BSpringnote) May 28, 2014
Military trucks moving east along Dnipropetrovsk-#Donetsk rail line. #Ukraine pic.twitter.com/Dm3APugcHO
— Sabra Ayres (@babraham) May 28, 2014
In front of DPR, militants blindfolded this man, beat him unconscious, put him in back of car and drove off. pic.twitter.com/kipCZ6YcfY
— Isaac Webb (@isaacdwebb) May 28, 2014
I check out and this happens. Stay safe all. MT @ChristopherJM: Several Vostok Battalion gunmen standing guard outside hotel Ramada
— Shaun Walker (@shaunwalker7) May 28, 2014
On Kievski Prospekt -barricades against #Ukraine army - antique shotguns. Mikhail:"they were good for my grandfather" pic.twitter.com/byDiRnHg2k
— Anshel Pfeffer (@AnshelPfeffer) May 28, 2014