More than 80 international human rights and aid groups are urging the U.N. Security Council to take real action against the continuing use of barrel bombs in Syria's grinding civil war.

The 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 has not followed up.

The new letter calls the council's expressions of concern "woefully inadequate."