A group of Syrian refugees including a couple with four young children detained in Turkey say they were tricked into being deported from Greece last month without having their asylum claims processed.

In the first case of its kind since the start of the EU-Turkey migration pact, at least eight Syrians were allegedly sent back to Turkey in October despite lodging asylum claims in Greece.

The allegations weaken the legal basis for the EU-Turkey deal, which was reached in March on the assumption that all refugees arriving in Greece would have access to a fair asylum procedure.

According to documents shared by the applicants with the Guardian, the refugees were initially given the chance to apply for asylum after landing in Greece on 9 October.

But 11 days later, before their applications were fully processed, they say they were tricked by EU and Greek officials into returning to Turkey.

“I never knew I was [going to be] deported to Turkey,” said Lawand Haji Mohamad, a 33-year-old from Kobani, Syria, who arrived in Greece with his wife and four children, all aged under five.

Instead, police told his family that they were being flown to the Greek mainland. “The policemen said leave your dinner, get your stuff, we will take you to a police station for the night and [then] tomorrow morning to Athens,” Haji Mohamad said by phone from a detention camp in southern Turkey called Düziçi.

After boarding a plane with officials from the Greek government and the EU border agency, Frontex, the Syrians say they were not provided with any further information. The first time they had any inkling that they were being deported against their will was on arrival at Adana, Turkey.

“When we arrived and saw the Turkish flag we were shocked,” said Haji Mohamad. “We trusted the police but they tricked us and I don’t have any idea why.”

For nearly a fortnight since, they have been detained in Düziçi. “We are in a very bad situation,” a second Syrian from a different family said by telephone from the camp. “The children are all very ill; the little ones have breathing problems. Please help us.”

Some of the Syrians are from Kobani, the Kurdish town that was briefly the centre of western attention after it was largely destroyed in battles with Isis.

In an emailed statement, the Greek government said it was examining the claims. A Frontex spokesperson confirmed that EU officials were onboard the flight, but said Frontex could not be blamed for any mistakes. “All return decisions are issued by the national authorities,” said the spokesperson. “Our role is to provide means of transport, trained escorts, translators and medical personnel.”

The UN refugee agency says it is concerned by the situation. Amnesty International fears the case could be part of a wider attempt to remove Syrians from Europe “at any cost”, in contravention of EU and international law.

The EU-Turkey deal, coupled with the closure of a humanitarian corridor through the Balkans, has seen migration flows to eastern Europe fall drastically since March. But Greek and European officials are worried that the numbers may surge again if too few refugees are deported back to Turkey.

John Dalhuisen, Europe director for Amnesty, said: “This is at best incompetence, and at worst a cynical attempt by authorities, under ever-growing pressure from the European Union, to remove Syrian refugees from the country at any cost. It needs to be urgently investigated, the refugees permitted to return to Greece and their relocation to other EU member states considered.”

While these are the first Syrians to be allegedly deported without recourse to the asylum process since the EU-Turkey deal, at least 12 people of other nationalities are believed to have been deported by mistake in April.

The 12 were never allowed to return to Greece, but the Syrians currently detained in Düziçi still dream of being given a second chance. “Is there any hope that we can return to Europe?” one asked in a recent text message.

Syrians are not at mortal risk in Turkey, which houses more refugees than any other country. But they are not given the rights they are allowed under international law. Despite recent legislative changes, the vast majority still do not have the right to work, and hundreds of thousands of Syrian children have no access to education.

Additional reporting by Apostolis Fotiadis