"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 on Monday.
Germany on Sunday reintroduced identity checks on people travelling within the passport-free Schengen zone, essentially reversing its open-door policy for Syrians.
Slovakia's interior ministry said on Monday that it had also renewed checks on its borders with Hungary and Austria. It said that more than 200 extra officers had been deployed at border crossings.
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.
"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: "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."
'Legal limbo'
By 8am on Monday, police said another 3,280 people had been held after crossing the border illegally, taking Hungary's 2015 total to nearly 195,000.
Many of the refugees flooding into Europe could be left in "legal limbo" as countries adopt different measures to address the situation facing the continent, the UN refugee agency warned.
UNHCR cautioned that "the combination of different, individual measures might create a situation where large numbers of refugees seeking in Europe the protection they are entitled to receive in line with international law, will find themselves moving around in legal limbo".
The agency insisted that "recent successive announcements of different border control measures by a number of European countries impacted by the refugee and migration crisis only underlines the urgency of establishing a comprehensive European response".
"This response must be based on the creation of effective reception centres, with the support of all parties concerned including UNHCR, to properly assist, register and screen people upon arrival in Greece, Italy and Hungary," it said in a statement late Sunday.
"People in need of international protection in accordance with international law should be relocated among all European Union countries based on a fair distribution mechanism."
European Union interior ministers will meet later on Monday in Brussels to discuss the distribution of the 160,000 refugees within the 28-member bloc.
Source: Agencies
