The U.N. Security Council on Thursday authorized a groundbreaking international investigative body that will be allowed to establish who is responsible for chemical attacks in Syria for the first time.

Until now, the U.N. and the global chemical weapons watchdog, the Organization for the Prohibition of Chemical Weapons, were only able to investigate attacks without laying blame on the government or rebels.

The new body is the result of rare co-operation between the United States and Russia on Syria's conflict, which is well into its fifth year.

No council member objected to a letter sent Thursday to Secretary-General Ban Ki-moon in support of the U.N.-OPCW joint investigative mechanism. In a statement, Ban's spokesman welcomed the approval and stressed "the speedy establishment and full functioning" of the investigative body.

The letter, obtained by The Associated Press, is signed by Russian ambassador and current council president Vitaly Churkin and authorizes Ban's recommendations for a three-member independent panel be backed by experts with the freedom to go anywhere in Syria to identify those responsible for attacks.

Syria's government denies using chemical weapons, but the United States and other Western nations contend it is to blame, especially for barrel bombs containing chlorine and other toxic agents dropped by helicopters. The opposition doesn't have aircraft.

Reports also have surfaced in recent months that the Islamic State group has used toxic chemicals.

Last month, the council unanimously approved a resolution giving a green light to establish the international body, which would operate for one year with the possibility of extensions.

U.N. disarmament chief Kim Won-soo on Wednesday acknowledged that "access will be a very challenging issue," especially with the Islamic State group controlling a large part of Syria.

Kim also said the investigative body to begin its work within weeks. One of the next steps is updating a co-operation agreement with the Syrian government, but "I don't expect a serious problem" there, he said.

The investigative body is expected to submit its first report within 90 days of being fully operational.