ANKARA (AFP) - An assistant professor at a Turkish university has appealed to the government for help after his three teenage sons left to fight with Islamic State in Iraq and Syria (ISIS) group Islamists in Iraq and Syria, the Hurriyet daily reported on Friday.

The professor, named only as Dr M. Sefik I., first lost his elder son, a dentistry student at Hacettepe University in Ankara, who on March 10 sent his parents a message asking them to pray for him before leaving Turkish territory.

The elder son then helped his 16-year-old twin brothers to cross the Turkish border to Syria, the paper added.

Turkey's National Intelligence Agency (MIT) has informed the family the three are believed to have first gone to Syria and then to Iraq to fight with the ISIS militants.

The parents have now appealed to President Recep Tayyip Erdogan, Prime Minister Ahmet Davutoglu and the MIT for help in urging them to come back.

"We taught them to be human beings. We do not know how they arrived at that stage," said the father, whose place of work and full name was not given by the paper.

According to Hurriyet, over 2,300 Turks are believed to have joined the ranks of IS militants who have taken swathes of Iraq and Syria up to the Turkish border.

Turkey has also been criticised for not doing enough to halt the flow of foreign militants across its borders but Erdogan said Friday 1,700 would-be foreign militants have been detained and deported.

"Turkey is fulfilling its task on this issue and will continue to do so," Erdogan said before leaving on a visit to Ukraine.