The Friends of Queslett Nature Reserve, in Great Barr, issued the warning after volunteers spotted adult footprints on the ice "going quite far out".
It comes after three boys aged 11, 10 and eight died after falling into Babbs Mill Lake in Solihull, with a six-year-old boy currently in a critical condition in hospital.
Meanwhile, shocking footage revealed a group of students risking their lives by walking on a frozen area of the Wyrley and Essington Canal in Brownhills.
A post from The Friends of Queslett Nature Reserve said: "The Friends of Queslett Nature Reserve would like to offer our deepest and heartfelt sympathies to the families of the children who died in a tragic accident of falling through the ice at Babb Mill Nature Reserve in Solihull.
"(On Tuesday) we paid a visit to the QNR and as expected the lake was frozen over but we were disturbed to see what looked like adult footprints on the ice going quite far out.
"We cannot say enough how treacherous the ice is. Just because the large birds can walk on top does not mean the ice can support the weight of an adult or child.
"Also, there were paw prints on the ice, far too big for a fox. So it is evident that people are letting their dogs loose on the ice as well. Again, the ice is very thin in places and the dog can fall through. Please be careful by the lake when it is iced over."
Patrols are carried out by the group when the temperature drops to keep an eye on the wildlife on the lake and ensure they're getting enough food. They have installed a make-shift sign up as well to warn people of the dangers of thin ice.
