Thursday's letter comes a week after more than 70 countries expressed "outrage" and demanded that the council prevent Syria's air force from using the weapons, which are prohibited by international humanitarian law because of their indiscriminate nature.
The council meets informally Friday to explore how the international community should respond as Syria's fighting is deep into a fifth year with well over 200,000 killed.
The council more than a year ago called for an end to the use of barrel bombs, but it has not followed up.
