German Interior Minister Thomas De Maiziere said in a statement Saturday that despite the effective closing of the West Balkan route to refugees, EU states still needed to protect themselves from a possible new migrant influx through other routes.

"Member states must continue to have the flexibility and option to conduct border controls at their internal borders in cases when it is necessary," the statement said, adding such measures were required to guarantee a certain level of safety.

The EU Schengen area's 26 countries allow free travel across their borders without the need for passports. But some countries were given permission to temporarily reinstate border checks, which is due to run out on May 12.

A German government official said the request for a six-month extension would be send to Brussels on Monday in a joint letter from Germany, France, Austria, Belgium, Denmark and Sweden.

The six countries have all clamped down on border entry as the continent battles its biggest migration crisis since the end of World War II. Germany took in more than a million refugees last year, but the numbers of arrivals has slowed significantly recently.

EU review of Greece

Earlier this month, Austria also announced tough new refugee controls, along with plans to build a 370-meter-long (1,200-foot-long) fence at the Brenner Pass in the Alps to protect against a sudden migrant influx, much to the annoyance of its neighbor Italy and the EU.

Brussels is expected to review Greece's efforts to protect the EU's external borders on the same day as the current controls expire. Analysts think a negative assessment would strengthen the case for an extension of the border checks.

A European Commission source told the Reuters news agency that Brussels was likely to green-light an extension, which will allow EU states to continue border checks until November.

Meanwhile, German Chancellor Angela Merkel made it clear she was against any attempt to allow long-term border controls, saying she would fight to uphold EU citizens' right to freedom of movement and residence within the bloc.

In her weekly video podcast, she urged EU member states to avoid seeking national solutions to European problems.