The Kremlin's approval of the joint inquiry by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW) came in the firm of a letter from Russia's ambassador to the UN Vitaly Churkin to UN Secretary-General Ban Ki-moon on Thursday.

The secretary-general's press office confirmed that it had received the letter and issued a statement saying that he had welcomed it and would, "without delay, undertake all steps, measures and arrangements necessary for the speedy establishment and full functioning" of the inquiry.

The start of the investigation, approved by the UN Security Council in August, had been delayed by objections from Moscow, which had sought to have the scope of the inquiry expanded to include alleged attacks by Islamic State in neighboring Iraq. Unnamed diplomats also told the AFP news agency that Russia had voiced concern about how the investigation would be financed and had sought guarantees that the sovereignty of its longtime Syrian ally would be respected.

Russia's concerns appear to have been allayed by a September 9 letter from Ban to Churkin in which the UN chief pledged to "expeditiously consult" with Damascus on an agreement governing how the mission will function and that there would be "reasonable grounds" for its demands for access.

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The Reuters news agency cited unnamed UN diplomats who said the Kremlin appeared to be concerned that the inquiry could end up confirming Western and rebel allegations Syrian President Bashar al-Assad's forces had repeatedly used chemical weapons, including deadly sarin gas.

Western governments have expressed the hope that the investigation will assign blame to specific individuals that could be used in possible future war crimes trials.

2013 agreement

Syria agreed to destroy its arsenal of chemical weapons in 2013 in an effort to avoid US air strikes after a sarin gas attack that killed hundreds of civilians. The OPCW has since found that chlorine has been "systematically and repeatedly" used in the Syrian conflict, but it is currently not mandated to lay blame.

A separate UN investigation had previously determined that sarin had been repeatedly used in Syria, but it too was barred from assigning blame.

Both the Syrian government and rebels have denied using chemical weapons in the conflict, which is well into its fifth year.

pfd/jil (Reuters, AFP, AP)