A memorandum signed by Russia, Iran and Turkey this week is the latest in a string of cease-fire proposals over the six years of conflict in Syria. On Friday, Russia announced that the plan was set to take effect at midnight. Here is a look at what it could mean for Syria and the major outside parties involved, including the United States.

What does the memorandum say?

The plan calls for a cessation of hostilities between Syrian President Bashar al-Assad’s government and the rebel groups seeking to topple it in four “de-escalation” areas in Syria. Russia, Turkey and Iran will act as guarantors of the cease-fire.

Russia and Iran have backed Assad during the conflict. Turkey has supported rebel groups and sent its own military to fight Islamic State militants as well as U.S.-backed Kurdish fighters in northern Syria.

The deal covers four areas: Idlib province in the north, where almost a million displaced people are packed among rebel forces dominated by al-Qaeda’s Syrian affiliate; Eastern Ghouta, a Damascus suburb where rebels threaten the government’s complete control of the capital; sections of the central province of Homs in which the patchwork of rebel forces includes a small contingent of al-Qaeda militants; and a stretch of Syria’s southern border with Jordan where the United States is supporting rebels against both Syrian forces and Islamic State militants.

The plan bans the use of weapons by all sides in the de-escalation zones, including “aerial assets” — a reference to the Syrian government’s air force. It also calls for “unhindered” humanitarian access, measures to restore infrastructure such as electricity networks and the creation of conditions that will allow refugees to return.

Critically, the plan allows for the guarantors to continue fighting not just the Islamic State but also al-Qaeda, which is allied with some rebel groups in the proposed safe zones. Previous cease-fires failed for this reason, as Syrian and Russian jets continued to bomb civilians in rebel-held areas.

[The Russia-Turkey-U.S. tussle to save Syria will still get very messy]

Are these no-fly zones?

It appears so, but there may be enormous loopholes. The ban does not apply to attacks on rebel groups who have not signed on to the agreement, and it specifically exempts attacks on al-Qaeda and Islamic State forces.

Russia and Syria have said they will not use air power in the designated zones, but if their forces claim the zones are being used as bases for militant activity, the ban can be waived.

Have the Syrian government and its opponents agreed to the plan?

Assad’s government said Wednesday that it supported the Russian initiative and was committed to “not shelling” the cease-fire zones. A spokesman for rebel groups who attended the talks in Kazakhstan expressed doubt Friday about the agreement, including with regard to Russia’s ability to force Iran and the Syrian government to respect a truce with the rebels.

More importantly, there has been no unified response from the fractured rebel factions, including some who were not invited to the talks. Several said that they supported any attempt to stem the bloodshed but that they doubted Russia’s intentions. Others, including a U.S.-based faction, said they would never accept Iran as a guarantor of any agreement.

How do Iran and Turkey feel about it?

The Iranian government has yet to issue a formal statement. Iran has previously resisted Russia’s cease-fire efforts in Syria and, along with its proxy forces in Syria, is pressing for the military defeat of Assad’s foes.

Turkey, which hosts more than 3 million Syrian refugees, has been calling for the establishment of no-fly zones in Syria since early in the war, but mostly along its southern border. The memorandum appears to allow the government of Turkish President Recep Tayyip Erdogan to continue its recent military offensives against Kurdish fighters in northern Syria whom it sees as an arm of Turkey’s Kurdistan Workers’ Party, or PKK, which Turkey has classified as a terrorist group.

Why is Russia supporting it?

Throughout the Syrian conflict, Russia has opposed no-fly zones that Western nations have called for but never agreed to enforce — most recently in October, during the bombardment of Aleppo.

But with the Syrian government’s territorial advances in the past year and the Trump administration’s apparent willingness to collaborate on counter-Islamic State measures, the Kremlin has appeared more amenable to a political solution to the conflict — on its own terms.

The zones may “help ensure Syria’s government control over the areas it seized,” said Maxim A. Suchkov, an expert on the Russian International Affairs Council. Because Moscow is monitoring the agreement, he said, it is unlikely to “impede its own military actions on the ground.”

The plan will play to Russia’s desire to appear as a peacemaker, Suchkov said. “So far, it looks like a political move serving to score Moscow some points.”

What does the United States think about all this?

It’s not clear yet. In January, President Trump said he would “absolutely do safe zones in Syria” to stem the flow of refugees to Europe, although it is unlikely he meant as part of a Russian plan with Iranian backing.

But when Trump and Putin spoke this week, a White House account of the call said the “conversation was a very good one, and included the discussion of safe, or de-escalation, zones to achieve lasting peace for humanitarian and many other reasons.” Putin said he believes he has White House backing for the plan, and the White House had not come out against the initiative as of Friday evening.

There are some unresolved differences, including the question of whether U.S.-led coalition aircraft may be barred from the de-escalation zones, as a Russian official claimed Friday. The Pentagon said Friday that Moscow had not officially notified it of any shift affecting air operations in Syria.

“We have not changed or altered our operations in any way,” said Capt. Jeff Davis, a Pentagon spokesman.

Louisa Loveluck in Istanbul and Karen DeYoung and Missy Ryan in Washington contributed to this report.

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