GENEVA - 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 said 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.

Faced with what the bloc has called its worst refugee crisis since World War II, German Chancellor Angela Merkel and French President Francois Hollande will hold talks in Berlin on Monday in a bid to give a fresh impetus to the EU's response in dealing with the situation.

Many of the migrants -- overwhelmingly Syrian -- said they had no choice but to make the long and uncertain journey taking them through Turkey and then to Greece before reaching Macedonia, then Serbia.

"It was hard to remain there, my children had no future," said 30-year-old Talal Din from Syria.

"I lost my job, I lost everything," he said. "I am an engineer, my wife is a teacher. We had no income, no jobs, nothing," he said, after crossing into Serbia early on Monday.

An AFP journalist at the Greek border village of Idomeni said hundreds of people were continuing to cross over to Macedonia on Monday.

Across the frontier at the entrance to the Macedonian town of Gevgelija, about one thousand people were waiting for trains to take them to the Serbian border, according to AFP journalists at the site.

Some 450 others had already left Gevgelija on Monday morning on buses, they said.

Nearly 340,000 migrants have arrived in the EU in the first seven months of this year, according to the bloc's Frontex border agency.