More than 70 countries have signed a letter demanding that Syria stop indiscriminate aerial attacks that have killed thousands of civilians and urging the U.N. Security Council to prevent the Syrian Air Force from future attacks including dropping barrel bombs.

The letter said Syrian air force helicopters repeatedly bombarded heavily populated areas in and around Aleppo, the country's largest city, in recent weeks leaving hundreds dead and dozens of others wounded.

The letter was sent Thursday to Secretary-General Ban Ki-moon and the presidents of the Security Council and General Assembly. It was drafted by the ambassadors of Belgium, Luxembourg and the Netherlands and the signatories include the United States, Britain and France — but not Russia and China, the two other veto-wielding members of the Security Council.