Greek riot police deployed to the border with Turkey to hold back thousands of migrants trying to push through may be using potentially lethal tear gas, a report said Thursday.

Investigative website Bellingcat said expended gas cannisters with pointed tips had been found in the vicinity of protests by asylum-seekers at the border between Greece and Turkey.

It also posted a picture apparently showing a helmeted man standing behind Greek riot police, loading such a cannister into a tear gas launcher.

"Unlike normal tear gas rounds, which have a limited range and would be unlikely to cause significant injuries, these long-range munitions... usually have significantly more kinetic energy than normal tear gas rounds," the website said.

"The combination of greater kinetic energy and a pointed tip make this kind of round potentially lethal to anyone hit by it," it said, adding that similar rounds have caused "serious injury or death of scores of protesters in Iraq".

Turkey has already accused Greece of firing live rounds against migrants, claiming injuries and deaths among those on the Turkish side of the border.

Athens has dismissed the allegation as "fake news".

Speaking in Brussels on Wednesday, Greece's deputy migration minister Georgios Koumoutsakos denied that live rounds had been fired near migrants, saying it was just "probably some rubber bullets".

In an effort to curb the influx, which began after Ankara said last week it would no longer stop refugees from entering Europe, Athens has suspended asylum procedures and reinforced its border forces.

The Greek government said border guards had prevented nearly 7,000 attempted entries over the last 24 hours, and nearly 35,000 over the last five days.

Among 24 people arrested since Wednesday, most were from Afghanistan and Pakistan, it said.