PRESEVO, Serbia - At least 2,000 more migrants flooded overnight into Serbia in a desperate journey to try and go on to Hungary, the door into the European Union, a UN official said Monday.

At least 7,000 people - mostly refugees from the brutal war in Syria - have been registered so far in the last days in overwhelmed Serbia as Europe's worst refugee crisis in half a century rapidly worsens.

All of them entered Serbia from Macedonia, where police on Saturday re-opened the border with Greece after spending three days trying to hold back the streams of migrants, when hundreds braved barbed wire fences and stun grenades to force their way through.

"The latest developments in Macedonia have led to a congestion and we now have tens of thousands of refugees who have entered Serbia from Macedonia," Davor Rako, a local official for the UN refugee agency told AFP in the southern Serbian town of Presevo. He said about 2,000 more migrants had registered at the border village of Miratovac, where Serbian authorities and the UNHCR have set up a reception centre with eight huge tents.

Buses were being laid on to nearby Presevo where police hand out officials documents and help migrants find their way towards their next destination, the border with Hungary.

Unlike Serbia, Hungary is an EU member state and therefore a popular crossing point into the bloc, although the country is currently building a four-metre (13-foot) barbed wire fence along its 175-kilometre border to stop the influx.

Meanwhile, 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.

Meanwhile, two people drowned and five were believed missing when a dinghy carrying migrants capsized off the Greek island of Lesbos on Monday, as many hundreds continued to head to Greek territory from nearby Turkey.

The Greek coastguard said it had rescued eight people on the northern coast of the island early on Monday, and recovered the bodies of two men.

Survivors told authorities that about 15 people were in the inflatable dinghy when it overturned. A search was underway in the area for survivors, a spokesman 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.