"At the moment, there is no prospect of a sustainable and significant reduction in the number of non-EU citizens streaming into the territory of the Federal Republic," the Interior Ministry statement read.
Such a reduction would be the only condition for lifting controls on Germany's borders, including its border with Austria, which - along with the European Commission and the rest of the countries in the Schengen free-movement zone - has agreed to the latest German move.
Nato ships head for Aegean
News of the extension to the controls came on the same day as Nato confirmed it would send ships to the Aegean sea between Turkey and Greece to combat people-smugglers bringing refugees into the EU.
NATO "is now directing the standing maritime group to move into the Aegean without delay and start maritime surveillance activities", Secretary-General Jens Stoltenberg told a press conference after alliance defence ministers backed the move.
NATO Standing Maritime Group 2, currently under German command, comprises three ships and allies had promised more so it can carry out "reconnaissance, monitoring and surveillance in the Aegean Sea," he said.
The military alliance had debated the move in response to a joint request from Germany, Turkey and Greece to step in.
"These are established criminal networks that extort and earn millions [of Euros] from these people and are happy to accept that thousands of them drown," German Defence Minister Ursula von der Leyen said in Brussels on Thursday.
Von der Leyen added that she did not believe a parliamentary mandate would be necessary – as is usual with military deployments – for German ships to take part.
She argued that the deployment was a simple maritime monitoring mission within Nato territory.
But the Nato plan is likely to face vocal opposition within Germany.
"The planned surveillance of the Aegean by Nato and forwarding data to Turkey is perfidious," refugee organization Pro Asyl said in a statement on Thursday.
They added that EU plans to keep refugees in Turkey – largely pushed by Chancellor Angela Merkel - were wrong, as the country was not safe under the definitions of the Geneva Convention on Refugees, and is also already struggling with 2.5 million refugees within its own borders.
While the sea crossing from Turkey to Greece is one of the shortest water routes into the EU, it remains dangerous.
The UN High Commissioner for Refugees (UNHCR) reports that 74,725 people arrived in Greece by sea between January 1st and February 9th – more than 44,000 of them in Lesbos, the largest of the Greek islands off the Turkish coast.
Syrian refugees accounted for 45 percent of those arrivals, with 29 percent coming from Afghanistan and 17 percent from Iraq.