"A lot points to the idea that this year we won't take on 800,000 refugees as the Interior Ministry has predicted, but a million," Gabriel wrote to Social Democratic Party (SPD) members in a letter seen by dpa.
But an Interior Ministry spokesman told Bild that the official estimate remains 800,000 asylum applications in this year, although he added that "it's in the nature of a forecast that it's only a forecast."
Gabriel's comments came on the same day that Finance Minister Wolfgang Schäuble repeated his insistence that Germany could and must manage the crisis in an interview with the Frankfurter Allgemeine Zeitung.
"Of course we won't solve the problems of the seven billion people in the world here at home in central Europe," Schäuble said. "But we won't turn away people who come to Europe because of terrible need. And we can't allow them to drown in the Mediterranean."
Schäuble added that "the more Germany does its part to solve the problem of refugees, the more other countries will be prepared to do the same".
But in Bavaria – which has been the front line of the refugee crisis for the country, with thousands arriving by train at Munich main station from Hungary in recent weeks – leaders insisted that help must come immediately from Germany's European neighbours.
"Every reasonable person" could see that things couldn't continue the way they had, Bavarian minister-president Horst Seehofer said before a meeting of the state's Christian Social Union (CSU) governing party.
Bavaria is calling for a fair sharing-out of refugees among EU countries, better control of the Union's external borders and more support on the ground in accommodation centres.
And the Free State leader called on Germany's federal government to at least double the money available for helping refugees and to improve the sharing-out of new arrivals among the country's federal states.
Seehofer added that "we remain a humane and helping state" which only needed a breather from new arrivals "to bring order back to the whole system" of refugee uptake.
Border controls were likely to remain in force for several weeks, Bavarian interior Minister Joachim Hermann said on Monday, adding that the stricter controls were necessary to block arrivals who were not genuine refugees.
"Border controls don't mean that no more refugees are coming to Germany... but the thousands and tens of thousands moving through central Europe... could no longer be accepted," he said.
Herrmann and federal Transport Minister Alexander Dobrindt added that in future, special trains would be laid on to bring refugees to Germany to better manage the flows of people.
"This gives us the chance to manage the trains and also control where they arrive," Dobrindt said, adding that it would make the police's job easier.
"There will certainly be some [trains] arriving in the coming days and weeks," Herrmann said.