"There are many signs that Germany this year will take in not 800,000 refugees, as forecast by the interior ministry, but one million," he wrote to members of his centre-left Social Democratic party.
Germany, buckling under the pressure of the wave of asylum-seekers, reintroduced identity checks Sunday on people travelling within the passport-free Schengen zone, essentially reversing its open-door policy to Syrians.
Gabriel stressed that "everyone who applies for asylum on German soil is allowed to stay here until a decision is reached. We are not changing that.
"The aim is however to maintain control of the borders in an unforeseeable and exceptional situation and to return to an orderly handling of refugee policy.
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"This includes a clear signal to our European partners that Germany, even if we're ready to make a disproportionate effort, can't go it alone and accept all the refugees."
He added that "Germany is strong and can do a lot. Still, in the last few days we have seen that, despite our best intentions, our capacity to take in people is reaching its limits -- especially in view of the speed of the refugee flow."
He said all state ministers had signalled that their capacity to accomodate the newcomers was nearly depleted.
"Despite many discussions with our European partners, we haven't succeeded so far in finding a collective solution to the refugee crisis," said Gabriel.
"No country can handle accepting and accommodating the refugees alone. It is necessary that we make this clear to our neighbours."