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.

The agreement, which a Russian delegate said would go into effect Saturday, 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 backing rival sides in the conflict signed the agreement at Syrian talks in Kazakhstan on Thursday, some members of the Syrian opposition delegation shouted in protest and walked out of the conference room in Astana, the Kazakh capital.

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 at the gathering.