Egypt government unlawfully expelled two Syrian refugees back to the war-ravaged country last week, and two Palestinians from Syria now risk being deported, the watchdog Human Rights Watch said on Saturday.

A Palestinian man and his son currently being held at Cairo airport were apparently refused entry to Egypt and would face "indiscriminate violence and possible persecution if returned to Syria," the New York-based group said.

"Egypt may have a right to detain people temporarily or investigate them on grounds of false documentation, but it may not under any circumstance return them to Syria," said Bill Frelick, Refugee Programme director at HRW.

"Egypt is obligated under international law not to return anyone, regardless of status, to a place where they would be persecuted."

Cairo airport officials deported two Syrians to Damascus on January 13 in violation of Egypt's obligations, although both had arrived in Egypt with valid passports the day before, the watchdog said.

Another attempted deportation of 13 Syrians in mid-December was stopped by immigration officials at the last minute.

The men had been in prison and were slated for deportation, but were released after their lawyer and the UNHCR intervened.

Human Rights Watch said that while Egypt has provided a safe haven for more than 13,000 Syrian refugees, recent cases raise concerns that others may be at risk of deportation.

"At a time of great conflict and tragedy in Syria, we call on Egypt and all countries not to return any residents of Syria, including Palestinians, to Syria," Frelick said.