Greece's government has come under pressure over its migration policies after footage emerged allegedly showing Greek coastguards expelling migrants by setting them adrift in the Aegean Sea.

The images, published by the New York Times, prompted calls by the UN High Commissioner for Human Rights for an independent enquiry.

The controversy, just one instance of accusations against the Greek government over illegal refugee expulsions, hit in the final days before a general election in which conservative Prime Minister Kyriakos Mitsotakis is facing a challenge from leftist former premier Alexis Tsipras.

A harsh anti-immigration policy has been a key theme of Mitsotakis's election platform. Earlier in the campaign, the prime minister traveled to the land border with Turkey where he said he would extend a five-meter-high fence to stop migrants getting into Greece.

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The footage in question was shot by a human rights activist on Lesbos last month.

In it, a group of asylum seekers including a baby, are driven in a white van to a "small cove spot with a wooden dock at the southern tip" of the island where they are taken out to the Aegean waters on a speedboat.

The migrants are then put on "a black inflatable life raft and set adrift", the New York Times wrote, adding that about an hour or so later, Turkish coast guard boats arrived to rescue them.

The report added that this was the April 11 rescue of "12 irregular migrants on the lifeboat that was pushed back to Turkish territorial waters by Greek assets." 

The New York Times said it had tracked down the migrants at Izmir detention facility where they recounted their ordeal.

Greece's migration ministry declined to comment on the issue, while the UN High Commissioner for Human Rights described the footage as "disturbing," calling for an investigation and closer monitoring of the border area.

"Everyone has the right to be protected from such treatment," said Ravina Shamdasani, spokesperson for the commissioner. "An independent, effective investigation is crucial."

"We remain seriously concerned about continued and systematic pushbacks at the Greece-Turkey border, which violate the prohibition of collective expulsions and the principle of non-refoulement," she added.

The commissioner backed "establishing an independent and effective border monitoring mechanism that would investigate allegations of violence at borders in collaboration with civil society," she said.

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