An EU border agency probe into allegations of illegal migrant pushbacks on the Greece-Turkey maritime border has proven inconclusive, according to a report seen Wednesday by AFP.

The report by a working group appointed by Frontex management, which was shared by a source close to the investigation, said it was "not able to clarify completely" the details of five incidents of alleged rights violations.

But the working group did point to "deficits and the need for improvement of the reporting and monitoring system" in place.

"The existing legal framework only offers limited options for Frontex for action in the event of reported and established legal violations," the report stated.

The investigation will be the main topic at an extraordinary meeting of the Warsaw-based agency's board on Friday.

The working group also emphasised the "difficult circumstances of conducting border police measures at the EU external maritime border in the Aegean Sea faced by all stakeholders".

It concluded that action by Frontex chief Fabrice Leggeri to halt operations in the Aegean Sea "would not be justified".

Pushbacks prevent asylum-seekers making refugee claims and if practised indiscriminately against a group of migrants can constitute "refoulement", a violation of core EU human rights laws and the 1951 Geneva Convention.

Leggeri has been under pressure for weeks over the allegations as Frontex takes on a greater frontline role in patrolling EU borders.

MEPs and activists have called for him to resign over the operations, but he has refused to, insisting the agency is key to the fight against human trafficking.

Created in 2004, Frontex has become the first EU agency to build up a standing, uniformed force. It aims to have 10,000 border guards by 2027.

Meanwhile, the agency is also being investigated by OLAF, the independent EU corruption watchdog.