Seventy countries across the planet united Thursday to express outrage over the Syrian regime's systematic use of barrel bombs and to demand an end to the deadly, indiscriminate attacks.

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.

UN peace envoy Staffan de Mistura has extended the latest talks until July, and the 70 countries urged all parties to work towards a political solution and a "genuine political transition."

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 Syrian President Bashar al-Assad.

The 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.

Neither China nor Russia made the list.

Barrel bombs are crudely made, non-guided missiles made out of large oil barrels or gas cylinders or empty water butts, filled with powerful explosives and scrap metal to exact maximum damage.

The Syrian regime denies that the weapons exist.