Turkey on Monday called on members of the U.S.-led coalition against Islamic State to provide air support for Turkey-backed troops laying siege to the Syrian town of al-Bab, held by fighters from the group.

"The international coalition must carry out its duties regarding aerial support to the battle we are fighting in al-Bab. Not giving the necessary support is unacceptable," President Tayyip Erdogan's spokesman, Ibrahim Kalin, told a news conference.

Rebels supported by Turkish troops have laid siege to the town for weeks in the "Euphrates Shield" operation launched by Turkey nearly four months ago to sweep the Sunni hardliners and Kurdish fighters from its Syrian border.