Poland's Ministry of National Defence commented on the matter on Friday. "The case is being analysed in details by our services," the ministry said.
According to Onet's report, the information may be extremely valuable for foreign intelligence services.
"Our source, a specialist in cybersecurity, said the database had already been downloaded by users from more than ten countries, including Russia and China," Onet wrote. "We have confirmed the leak in two independent sources connected with special forces," it added.
The leaked data include information on equipment purchased by Poland in the United States, Germany and Israel, the website said.
Onet wrote that the leak likely originated from the army's logistics planning unit in Bydgoszcz, north-central Poland.
The ministry also said that it is investigating "whether there was a leak and whether the leaked file had been in fact produced by Poland's Armed Forces. It added that one of the analysed scenarios was whether the document had been just a software test file or a non-classified public tender file.
In another press release published in the early afternoon on Friday, the ministry said that the leaked list contained non-classified information only.
"The list comprises only publicly-available information," the ministry said. "The information can be obtained on the basis of open public tender announcements."
The ministry also said that "the publication of the data does not constitute a threat to state security or the operation of Poland's Armed Forces."
However, Poland's main opposition grouping, the centrist Civic Coalition (KO), has demanded that Defence Minister Mariusz Blaszczak resign. KO has also said that the government should provide a detailed explanation to the public on how the leak happened and who was responsible.
Blaszczak quickly took to Twitter to respond, saying that no classified information has been leaked. "I recommend that the politicians of the 'total opposition' sober up and drink a glass of cold water." (PAP)