BEIRUT, Lebanon — Russia, Iran and Turkey signed a memorandum on Thursday to create four “de-escalation zones” in Syria, to reduce bloodshed in a war now in its seventh year, but many questions remained about the plan.

Presented at talks in Astana, Kazakhstan, the memorandum was the most ambitious of recent proposals, but it was not signed by the Syrian rebels or government. Rebel representatives said it left too many loopholes for the Syrian military to continue what the rebels called indiscriminate bombings of civilian areas.

The memorandum calls for a pause in fighting, including government airstrikes, and for unhindered aid deliveries in and around the four main zones still held by rebels unaffiliated with the Islamic State. It also calls for all parties to fight jihadists like the Islamic State and the Qaeda-linked group once known as the Nusra Front.

The top United Nations envoy dealing with Syria, Staffan de Mistura, called the memorandum an “important, promising, positive step in the right direction.”