The leaders of Germany and France will meet in Berlin on Monday to seek a unified stance on efforts to tackle the biggest migrant crisis since World War II, as hundreds more people poured into Serbia in a desperate bid for a better life.

At least 2,000 more migrants entered Serbia overnight from Macedonia, which has declared a state of emergency over the massive numbers pressing into the country from the Greek border.

They are trying to reach Hungary, a member of the EU's open-border Schengen agreement, which has already registered over 100,000 asylum seekers this year and plans to finish its anti-migrant fence on the Serbian border by the end of August.

Hours ahead of the talks with French President Francois Hollande, German Chancellor Angela Merkel condemned "vile" violent protests against refugees, as anti-migrant sentiment reared its head over the weekend in the eastern city of Heidenau.

"The chancellor and the entire government condemn the violent rampages and the aggressively xenophobic atmosphere in the strongest terms," Merkel's spokesman Steffen Seibert told reporters after the clashes between police and far-right thugs protesting the opening of a new centre for refugees.

"It is vile how right-wing extremists and neo-Nazis try to spread their hollow, hateful messages. Those who act like the aggressors of Heidenau place themselves far outside the law," he said.

Germany, which expects to take in 800,000 asylum seekers in 2015, has been struggling like the rest of Europe to find a response to the unprecedented numbers arriving.

EU border agency Frontex said last week that a record 107,000 migrants were at the bloc's borders last month, with 20,800 arriving in Greece last week alone.

With many seeking to cross into Macedonia from Greece, Skopje closed the border for three days and police used stun grenades and batons to stop hundreds of refugees trying to break through barbed wire fencing, before apparently deciding to let everyone enter.

Austria's foreign minister Sebastian Kurz, who had travelled to the Macedonia-Greece border, called for an urgent new strategy to deal with the crisis.

"It's a humanitarian disaster, a disaster for the European Union as a whole, and there is a pressing need for us to focus on the situation in the western Balkans," said Kurz.

In Rome, Italian officials said the coastguard had rescued 4,400 migrants from 22 boats in the Mediterranean on Saturday alone in what was understood to be the highest daily figure in years.

"There has to be a new impetus so that what has been decided is implemented," a source in the French presidency said, referring to EU decisions taken in June to tackle the crisis.

"The situation is not resolving itself," the source said, adding that the decisions made by the EU "are not sufficient, not quick enough and not up to the task".

With asylum-seekers coming not just from war zones such as Syria but also from countries without military conflict in southeastern Europe, including Albania, Serbia and Kosovo, calls are mounting for a more unified approach in dealing with the influx.

France and Germany are both urging Brussels to compile a list of countries whose nationals would not be considered asylum-seekers except in exceptional personal circumstances.

Merkel is also travelling Thursday to Vienna, where she will meet with leaders of Balkan states including Albania and Kosovo to find out why "so many thousands of people are coming from these countries", according to Seibert.

France's and Germany's leaders will try to help fast-track the setting up of reception centres in overwhelmed Greece and Italy -- two countries that have borne the brunt of the crisis -- to help identify asylum-seekers and illegal migrants.

"As long as these reception centres are not there and there is no internal solidarity within the EU, the return of migrants -- which will dissuade further new arrivals -- will not happen," the French source added.

Italian Foreign Minister Paolo Gentiloni has warned that the deepening crisis could pose a major threat to the "soul" of Europe.

"On immigration, Europe is in danger of displaying the worst of itself: selfishness, haphazard decision-making and rows between member states," Gentiloni told Il Messaggero.

Beyond the migrant crisis, Merkel and Hollande will later Monday tackle another issue pressing at the EU's eastern flank -- the Ukraine conflict.

The talks with Ukrainian President Petro Poroshenko come amid a resurgence of violence in the former Soviet state.

Hours ahead of the talks, Poroshenko accused Russia of sending three military convoys over the border into the separatist-controlled east.