Macedonian riot police on Friday fired stun grenades to drive back thousands of mostly Syrian refugees stuck in no-man's land between Greece and Macedonia, the latest flashpoint in Europe's escalating migrant crisis.

At least eight people were hurt near the Greek border village of Edomeni as the police in riot gear beat the migrants back with truncheons and threw the stun grenades, devices that produce a blinding flash of light and a huge noise to disorient their targets.

The grenades set off clouds of smoke, sending panicked refugees running for cover. Some were injured as they fell to the ground; one youth's face was covered in blood.

Rights group Amnesty International blasted the police response, saying it had received reports that officers fired into the air in "an unacceptable push-back in violation of international law".

Macedonia had on Thursday declared a state of emergency and sealed the border, stranding more than 3,000 mostly Syrian refugees who are hoping to travel onwards to northern Europe and start a new life.

But after tensions boiled over on Friday, Macedonia said it would allow a limited number of "vulnerable" migrants to enter and some 500 refugees were able to cross the border, mostly families with children and pregnant women.

In Brussels, the European Commission defended itself against charges that the EU is responding too slowly to Europe's biggest migration crisis since World War II.

Dimitris Avramopoulos, the EU's Migration and Home Affairs Commissioner, insisted the commission had been working "day and night" to offer funding and other support to states struggling to cope with the influx of migrants from the Middle East, Africa and South Asia.

German Interior Minister Thomas de Maiziere had said Thursday that it was "unacceptable that European institutions continue to work at their current slow pace" in finding a joint solution to the crisis.

- 'Help us' -

The numbers on the Greek-Macedonian border have been building for a couple of days, and Friday's clashes broke out as the crowd surged towards the police cordon, shouting "Help us".

There are only five portable toilets in the area, and a few volunteers providing aid. Amnesty warned that many of those arriving at the border need medical assistance, with some showing signs of war injuries.

Many of those stuck on the border between Greece and Macedonia said they hoped to eventually reach Germany or Sweden.

The migration crisis has sparked a bitter war of words in Europe, with some countries complaining that they are having to shoulder much more of the burden than others.

Tens of thousands of migrants have washed up on the shores of Greece and Italy after crossing the Mediterranean in over-packed boats.

Germany, as Europe's biggest economy, has become the refugees' top destination, with media reporting that the government expects to receive up to 750,000 asylum claims this year.

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

Europe's interior and foreign ministers will meet in mid-October to discuss how to respond to the huge influx of migrants and refugees arriving at its borders.

The meeting, to be followed by further talks in Berlin, will pave the way for broader discussions at an EU summit in Malta in November which will also be attended by African leaders, France's Interior Minister Bernard Cazeneuve said Thursday.