(This version of the story corrects the second paragraph to clarify Idlib is not the first de-escalation zone)
A de-escalation agreement in Syria takes effect at midnight but Russia's air force will continue strikes against Islamic State elsewhere in the country, Russian news agencies cited the Defense Ministry as saying on Friday.
The largest of the four de-escalation zones is in northern Syria and includes Idlib province and adjoining districts of Latakia, Aleppo and Hama with a total population of over 1 million, the ministry said.
Iran and Turkey agreed on Thursday to Russia's proposal to establish the de-escalation zones.
But details of the memorandum the three guarantors signed were sketchy, while the main Syrian opposition group said it lacked all legitimacy.
The zones appear intended to halt conflict in specific areas between government forces and rebels, and would potentially be policed by foreign troops.
The fact that the de-escalation agreement was supported by the United Nations, the United States and Saudi Arabia guaranteed its implementation, the RIA and Interfax news agency quoted Deputy Defense Minister Alexander Fomin as saying.
In Washington, Pentagon spokesman Captain Jeff Davis told reporters the U.S.-led coalition in Syria had not altered its operations, but declined to comment on the de-escalation zones.