The country has triggered the "alarm" stage of an emergency gas plan to deal with shortages, Germany's economy ministry said.
"We must not fool ourselves. The cut in gas supplies is an economic attack on us by [Russian President Vladimir] Putin," Mr Habeck said, adding Germans would have to reduce consumption.
Mr Habeck said there would "hopefully never" be a need to ration gas for German industry, but he added: "Of course, I can't rule it out."
Germany has now moved to the second stage of its three-part emergency plan, which is triggered when there is disruption or very high demand for gas.
The government will provide €15bn (£13bn) of loans to try to fill gas storage facilities, and will start auctioning gas to industry to encourage big businesses to use less.
Moving to stage two of the plan puts more pressure on suppliers and network operators to balance out disruption by taking measures such as finding alternative sources for gas.