Europe's border security agency Frontex said Tuesday it had opened an internal inquiry over media reports that it was involved in illegal "pushbacks" of asylum seekers.

"We are looking into the accusations levelled by several news organisations related to our activities at Greece's external borders," agency chief Fabrice Leggeri said in a statement.

But the Warsaw-based agency said that "so far, no documents or other materials have been found to substantiate any accusations of violations".

EU home affairs commissioner Ylva Johansson also on Tuesday told German public radio Deutschlandfunk that any involvement in pushbacks would be "completely unacceptable".

"The director of Frontex must take full responsibility, investigate these cases and present an answer to what really happened," she said.

"Pushbacks" are incidents where refugees or asylum seekers are illegally returned across a border to a country where they could face persecution.

Several media including Germany's Der Spiegel last week alleged that some Frontex officials were implicated in pushbacks carried out by Greek border guards at sea.

Journalists said they had uncovered six cases since April when Frontex units at a minimum did nothing to stop refugee boats in Greek waters being returned towards Turkey.

A video from a June incident allegedly showed a Frontex boat blocking a refugee boat.

Greece's conservative government has always rejected claims of illegal pushbacks taking place at its borders -- a route for many migrants entering the EU.

The Frontex agency's statement on Tuesday said that earlier this year it had asked Greek authorities to investigate "two events near its islands in the eastern Aegean Sea".

"They found no proof of any illegal acts in one incident and are still looking into another one," it said.

"At this point of time, the still ongoing inquiry has not identified other suspicious cases than those already reported by the executive director to Greek authorities."

Frontex also said its operations in the region were complicated by a maritime border dispute between Greece and Turkey.