Russia said it’s ready to send peacekeepers to Syria after Turkey and Iran agreed on Thursday to Russia's proposal for "de-escalation zones" in Syria. The move, welcomed by the United Nations, has been met with scepticism from the United States as the so-called safe-zones will closed for warplanes of the United States and those of the U.S.-led coalition.

As Bloomberg reports, the three countries signed a memorandum on the creation of so-called de-escalation areas on Thursday after two days of talks in Kazakhstan that also included representatives of the Syrian government and rebel groups.

Opposition leaders distanced themselves from the plan, saying they can’t accept Iran as a guarantor of the truce and that they want “clear and tangible” guarantees the deal will be enforced.

The U.S. also expressed doubts, as State Department spokeswoman Heather Nauert said Thursday that the U.S. has “concerns” about the accord, “including the involvement of Iran as a so-called “guarantor,”’ and said Russia should do more to stop violence.

The four safe zones to be established in Syria will be closed for flights by US-led coalition warplanes, said the Russian envoy to the Astana peace talks, where the zones were agreed upon.

“Russia is ready to send its observers” to help enforce the safe zones, President Vladimir Putin’s envoy to Syria, Alexander Lavrentiev, told reporters in the Kazakh capital, Astana. “We believe the Syrian crisis can only be resolved through political methods.” “As for [the coalition] actions in the de-escalation zones, starting from now those zones are closed for their flights,” Aleksandr Levrentyev told journalists in the Kazakh capital.

The Russian Ministry of Defense notes that the deal on safe zones in Syria wil come into effect 21:00 GMT on May 5.

We wonder how long Washington will stand for a "no-fly-zone"?