The country's COVID-19 reproduction ("R") number, an indicator of how many others are being infected by one person with the virus, is beginning to go up, Niedzielski said on Monday.
The figure for Poland is now around 1, up from a low of 0.63 last month, according to data he shared on social media.
The "R" value is used as a way of measuring the rate at which coronavirus infections are growing or falling.
A value of 1 means that, on average, every person with the coronavirus will infect one other person.
A value of more than 1 indicates that the number of cases will increase, while a lower number means theoretically that a disease will eventually stop spreading.
A total of 2,881,491 people have tested positive for the coronavirus in Poland since the start of the pandemic, and 75,215 have died in connection with the COVID-19 respiratory disease so far, officials said on Monday.
Poland on Monday reported 67 new coronavirus infections and no new deaths related to COVID-19, inviting comments that the epidemic was tapering off in the country.
But public health authorities have warned in recent weeks that the number of COVID-19 cases in Poland could soar again in the autumn.
They have been urging people to get vaccinated before a potential fourth wave of infections.