BEIRUT -- Russia, Iran and Turkey agreed to establish "de-escalation zones" in Syria, signing on to a Russian plan under which President Bashar Assad's air force would halt flights over designated areas across the war-torn country.

It emerged Friday that the trilateral deal also seeks to ban U.S. aircraft from flying through the designated areas.

Russia's representative at the Syria talks said the "de-escalation zones" would be closed to military aircraft from the U.S.-led coalition. Alexander Lavrentyev's comments Friday came a day after talks in Kazakhstan where Russia, Iran and Turkey agreed to establish the zones.

Lavrentyev, whose remarks were carried by Russian news agencies, said U.S.-led coalition aircraft would be able to operate against Islamic State of Iraq and Syria (ISIS) militants in specific areas, but the "de-escalation zones" were now closed to their flights.

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The agreement is the latest attempt to reduce violence in the Arab country. But the full details of the proposal were not made available and prospects for its success appeared bleak.

As officials from the three countries that back rival sides in the conflict signed the agreement at talks in the Kazakh capital of Astana on Thursday, some members of the Syrian opposition delegation shouted in protest and walked out of the conference room.

The opposition is protesting Iran's participation at the conference and role as a guarantor of the agreement, accusing it of fueling the sectarian nature of the conflict that has killed some 400,000 people and displaced half the country's population.

"Iran is a country that is killing the Syrian people and the killer cannot be the rescuer," said Abu Osama Golani, a rebel commander who attended the gathering in Astana.

The walkout and the comments underline the huge difficulties of implementing such a deal. The Syrian 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.

A previous cease-fire agreement signed in Astana on Dec. 30 helped reduce overall violence for several weeks but eventually collapsed. Other attempts at a cease-fire in Syria have all ended in failure.

Sponsors of the deal hope that safe zones would bring relief for hundreds of thousands of Syrian civilians and encourage refugees to return. But officials have expressed skepticism, stressing that safe zones have not had an encouraging track record.

Asked earlier this year to comment on the idea of safe zones after it was raised by President Donald Trump, U.N. Secretary-General Antonio Guterres pointed to tragic examples such as Srebrenica, a U.N.-protected enclave in the Bosnian war where 8,000 Muslim men and boys were massacred in 1995.

However, U.N. Syria envoy Staffan de Mistura described Thursday's agreement as a "step in the right direction," expressing hope that the deal would boost U.N.-backed Syria peace talks in Geneva.

De Mistura said details of the Astana agreement would become clearer in the next two weeks, adding that the United Nations was "very supportive" of the deal.

U.S. State Department spokeswoman Heather Nauert said Thursday, before Russia said U.S.-led coalition flights would also be restricted under the deal, that the United States had "reason to be cautious" about the chances for success of the deal, though it appreciates the efforts by Russia and Turkey to help lower violence in Syria. She added that U.S. has concerns, including about Iran's "guarantor" role in the Kazakhstan deal.

"Iran's activities in Syria have only contributed to the violence, not stopped it, and Iran's unquestioning support for the Assad regime has perpetuated the misery of ordinary Syrians," she said.

The U.S. was represented at the talks in Kazakhstan though Nauert said it was neither a "direct participant" nor a party to the agreement.

It was unclear whether U.S. officials were aware of the stipulation banning coalition flights in the "de-escalation zones" before it was announced on Friday.

Along with Mr. Trump, the presidents of Russia and Turkey have recently supported the idea of creating safe zones in Syria.

Meeting in the Russian resort town of Sochi on Wednesday, Recep Tayyip Erdogan and Vladimir Putin expressed hope the Syrian government and rebels would adopt this latest proposal to "de-escalate" the conflict.

Putin said Russian and Syrian government jets would halt flights over the specified zones if all sides respect the cease-fire.

The Kazakhstan agreement calls for delineating zones where front lines between the Syrian government forces and the rebels would be frozen and fighting halted. The zones include the provinces of Idlib, areas north of Homs, the eastern Ghouta suburbs outside Damascus, and an area in the south of the country, according to a statement.

Turkey's foreign ministry suggested the scope was wider and would include the whole of Idlib province; parts of Latakia, Aleppo and Hama provinces; parts of Homs province; parts of Damascus and the East Ghouta region; and also parts of the southern Daraa and Quneitra provinces.

It was not clear how a cease-fire over such a broad and logistically complicated area would be achieved, or whether international observers would be sent to Syria to monitor its implementation.

The head of Russia's delegation at the Kazakhstan talks, Alexander Lavrentyev, said the Syrian government would abide by the agreement, unless rebels inside those areas stage attacks.

"As of the sixth of this month all military operations will be ceased," the Russian official told journalists in Astana. "All Syrian flights over these areas will cease."

Lavrentyev said Turkey, Iran and Russia have agreed on the possibility of allowing international observers in case there is "unanimity" on that issue. He spoke at a press conference shortly after delegations of the three sponsor nations signed the agreement in the presence of de Mistura.

Military operations are likely to continue against members of al-Qaida-linked group known as the Levant Liberation Committee, which is active in areas where the fours zones are meant to be.

Iran and Russia have been among the strongest backers of Assad while Turkey is a main supporter of opposition groups that have been trying to remove him from power. Turkey and Russia are deeply entangled in the war in Syria, with each country having troops on the ground there.

Osama Abo Zayd, a spokesman for the Syrian military factions in Astana said the zones raise "a number of questions," adding that Moscow still has no answers on how to deal with any violations from its ally Damascus or from Iran, which has a number of fighters on the ground in Syria on the government's side.

"If this (negotiating) table does not lead to solutions, we will return to the gun," he said.

Separately, a Syrian monitoring group and an official with a U.S.-backed force said on Thursday that Kurdish-led fighters have captured all parts of a Tabqa, a northern town in Syria that was held by ISIS.

The Britain-based Syrian Observatory for Human Rights said ISIS fighters withdrew from Tabqa and the nearby dam after reaching an agreement with the Syrian Democratic Forces to leave.

However, Brig. Gen. Hussam al-Awwak, an Arab spokesman for the SDF, told The Associated Press on Thursday that no such agreement was reached but that a corridor was opened for the extremists to leave the dam, which he said they still control.

SDF has been on the offensive in Tabqa since mid-April.