70 countries across the planet united on Thursday to express outrage over the Syrian regime’s systematic use of barrel bombs and to demand an end to the deadly, indiscriminate attacks, according to AFP.

May 2015 was “reportedly the deadliest month of the Syrian crisis so far,” said a letter addressed to the president of the UN Security Council. The more than four-year war has killed 220,000 people.

Recent barrel bombings in and around the northern city of Aleppo had killed hundreds of civilians and wounded dozens “with many victims blown to pieces or burnt beyond recognition,” the letter said.

The attacks were “among the most brutal perpetrated since the start of the Syrian crisis,” it added.

Since the beginning of the war, “grisly and horrific” bombings of markets, hospitals, schools, places of worship and residential buildings have killed thousands, the countries said.

The indiscriminate use of weapons, including barrel bombs, is prohibited under international law and must cease, said the letter.

It called on the Security Council “to advance its efforts” on preventing the Syrian air force from carrying out barrel bombings.

Barrel bombs are crude weapons, made of containers packed with explosives and scrap metal, that are typically dropped from helicopters and are one of the regime’s weapons of choice.

Their use has been criticized by human rights groups because of the indiscriminate death toll they cause among civilians when dropped on residential areas.

Syrian President Bashar Al-Assad, however, has repeatedly denied using barrel bombs and has suggested in interviews that no such weaponry exists.

The 70 countries urged all parties to work towards a political solution and a “genuine political transition” after UN peace envoy Staffan de Mistura extended the latest talks until July.

Diplomats say that France has started consultations with its allies on the Security Council to table a specific resolution on barrel bombs in order to increase the political pressure on Damascus.

Since the start of the conflict, the Security Council has been frequently paralyzed by China and Russia, which exercised their veto rights to protect Assad.

Thursday’s letter, organized by Belgium, Luxembourg and the Netherlands, was signed by European countries, Canada and the United States, and regional powers including Saudi Arabia and Turkey, according to AFP.

Neither China nor Russia made the list.

AFP contributed to this report.