The Mountaineering Council of Scotland (MCofS) said wind, snow and freezing temperatures have made walking and climbing conditions difficult.
The warning comes ahead of the Easter weekend, traditionally one of the busiest times in Scotland's hills.
MCofS said mountain rescuers have "serious safety concerns" about the weekend.
Heather Morning, the council's safety adviser, said only people with adequate gear and knowledge of winter conditions should go out.
She said: "If you are heading above 600m this Easter you must take an ice axe and crampons.
"And rigid-soled boots must be worn to kick steps across old, hard snow patches.
"Don't be fooled into thinking that a pair of lightweight slip-on snow spikes will do the job. They won't.
"Crampons that are not securely clipped and strapped to your boots will roll off as soon as you leave flat ground."
Simon Steer, chairman of Scottish Mountain Rescue, added: "It's a sad fact that the beautiful, yet deceptive, conditions on the hills at Easter means that this is often a busy time for the volunteers in the mountain rescue teams.
"We hope that hill goers will heed advice of the MCofS to avoid getting into difficulties which can place both themselves and their rescuers at risk."
The Sportscotland Avalanche Information Service has rated the avalanche hazard for Creag Meagaidh as "high" and "considerable" for Glen Coe, Lochaber and Northern and Southern Cairngorms.
People have been asked not to go to Scotland's mountains unless they have the proper equipment and experience.