An agreement to set up "de-escalation zones" in Syria came into effect at midnight on Friday, but it will be at least another month before all the details are worked out and the safe areas are fully established, according to Russian officials.

The plan was agreed by Russia, Turkey, and Iran during Syria talks in Kazakhstan's capital, Astana, on Thursday, but sharply criticised by the Syrian opposition which rejected Iran's role as guarantor in any deal.

It envisions establishing four safe zones that would bring relief for hundreds of thousands of Syrian civilians and encourage refugees to return, Russian military officials said on Friday.

FOUR 'DE-ESCALATION ZONES' IN SYRIA: - The first one includes Idlib, as well as northeastern areas of Latakia province, western areas of Aleppo province and northern areas of Hama province. There are more than one million people in the zone. - The second one is in the north of Homs province. It includes al-Rastan and Talbiseh, as well as nearby areas controlled by the opposition groups. There are about 180,000 civilians in the zone. - The third one is eastern Ghouta, home to about 690,000 civilians. This zone does not include the area of Qaboun. - The fourth zone is located in the south of Syria and includes areas of Deraa and Quneitra provinces. Up to 800,000 civilians live there. Russian defence ministry

There were limited reports of bombing in northern Homs and Hama, two areas expected to be part of the "de-escalation zones," after midnight. There were no immediate reports of casualties.

Russia, Turkey and Iran are to enforce the zones, but Russian general staff official Sergei Rudskoi said that other countries could participate. He did not elaborate on which those countries might be.

The plan, which has not been published, calls for all aircraft to be banned from flying over the safe zones.

Syrian, Russian, Turkish and US-led coalition aircraft operate in overlapping areas across Syrian airspace.

According to media reports, the plan allows for Russia, Turkey and Iran to continue fighting ISIL, as well as al-Qaeda-affiliated groups inside the safe zones.

Previous ceasefires have collapsed as Russian and Syrian jets continued to hit civilians under the premise of targetting hardline rebels.

Syria's government has said that although it will abide by the agreement, it would continue fighting "terrorism" wherever it exists, parlance for most armed rebel groups fighting government troops.

Reacting to the announcement that the "de-escalation zones" will be closed to military aircraft from the US-led coalition, Pentagon spokesman Jeff Davis said the US military has "not changed or altered our mission in any way".

"These de-escalation zones, as I have seen them on the map, are in western Syria and not in areas where ISIS [Islamic State of Iraq and the Levant] is active," he said. "Our operations are focused on ISIS, which are farther east."

'Vague and illegitimate'

The latest round of Syrian peace talks in Astana was sponsored by opposition supporter Turkey and Syrian government backers Russia and Iran.

Yet, the de-escalation zones agreement was not signed by the Syrian government, nor the opposition.

Representatives of several rebel groups in Astana said they could not accept Iran as a guarantor of the deal.

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The High Negotiations Committee (HNC), the main Syrian opposition umbrella group, criticised the plan as vague and illegitimate.

The Riyadh-based HNC, which includes political and armed groups, cautioned against attempts to "partition the country through vague meanings of what has been called ... 'de-escalation' zones," in a statement on Friday.

Mohammed Rasheed, a spokesman for the Jaish al-Nasr rebel group, said Russia was "merely playing political games" and "making declarations".

He said rebels doubted Russian or Syrian government warplanes would stop striking rebel-held areas after the deal takes effect.

"This is not the first time," he added, referring to several mediated ceasefires that have unravelled in Syria's multi-sided conflict.

Al Jazeera's Stefanie Dekker, reporting from Moscow, said the Russian Ministry of Defence had set out some of the details that would be established next; including making specific maps of the zones, establishing coordinates, and setting up buffer strips and checkpoints which would allow civilians freedom of movement and the access of humanitarian aid.

"[Russia] did mention that, if there is any military activity, there could be repercussions. How exactly that is going to play out - we don't know the details," said Dekker.

Chris Doyle, the director of the Council for Arab-British Understanding, told Al Jazeera that the agreement is a step forward, "but there are so many spoilers in this".

Doyle said there are huge question marks over the commitment of various actors to reducing violence and over how the plan will be enforced. He said confidence measures and further agreements will be needed to back up the agreement.

"If you are going to make these areas in any way safe on the ground, you're going to need a significant military deployment and you're probably going to need third parties who will have the trust of all sides," said Doyle.

"We're nowhere near that sort of situation at the moment because nobody has been able to demonstrate that the violence is going to be able to recede to the sort of levels that are necessary."

"The United Nations, the United States and Saudi Arabia have welcomed the deal," Russian Deputy Defence Minister Alexander Fomin said in comments carried by state news agency TASS.

The US state department said that it supports "any effort" that can genuinely move towards peace but voiced scepticism about the new agreement's ability to create safe zones.

With the airspace closed over these so-called safe areas, they would become de facto no-fly zones. Calls for such a measure have been made numerous times by opposition members, but it has never been imposed until now.

Previous regions considered for no-fly zones were the border areas with Jordan and Turkey.

Syria's civil war, currently in its seventh year, has killed hundreds of thousands of people and has drawn in world powers on all sides.

The negotiations in Astana are viewed as complementary to broader United Nations-brokered talks in Geneva on a political settlement, but neither have yielded real progress as of yet.