His rejection comes after Russia, Iran and Turkey, who agreed to act as "guarantors" of the ceasefire, reached a deal on Thursday to create "de-escalation zones" across Syria in a bid to halt hostilities between government and opposition forces.
Syria's ally Russian Federation and regional power Iran have helped President Bashar al-Assad gain the military advantage against rebels fighting for six years to unseat him, and Moscow has led most of the recent diplomatic efforts to end the conflict. The SDF is often criticized by other Syrian rebel factions of being too close to the Syrian government.
"There are still logistical details that will be discussed in Damascus and we will see the extent of commitment to this agreement", al-Moallem said.
"We'll look at the proposal, see if it can work", Mattis told reporters traveling with him to Copenhagen, where he will attend a meeting of the USA -led coalition battling the Islamic State group.
"In this phase, we believe that what Kurdish citizens in Syria are doing by fighting Daesh is legitimate, in the framework of protecting Syria's territorial unity and sovereignty", he said, using the Arabic acronym for ISIL. "I think the global community is united in the sense of wanting to see ISIS put on its back foot".
He says armed Syrian opposition groups must separate themselves from extremist groups such as al-Qaida Syria branch.
Muallem said rebels involved in the process must help to clear areas they control of hardline militant groups, including the group formerly known as the Nusra Front which operates in the "de-escalation zones".
Even if the agreement is enforced, it is unlikely to end the conflict. Despite several rounds of United Nations -mediated negotiations in Geneva, the government and opposition remain at odds over Assad's future role in Syria.
Al-Moallem said the Syrian army's military priority now is to liberate areas in eastern Syria near the Iraqi border held by the Islamic State group.
Al-Moallem said the government's alternative to stalled negotiations has been the implementation of "reconciliation agreements" around the country.
The development is the latest in a series of population transfers in the war-torn country over the past year. However, the organization reported that only four buses had left the Damascus suburb of Barzeh, which has been under siege for the past month.
Over the past months, tens of thousands of people living in besieged areas around Damascus, Homs and Aleppo - Syria's largest city - have surrendered under similar agreements.
Meanwhile on Monday, the evacuation of fighters and their families from the eastern outskirts of Damascus began, according to the UK-based Syrian Observatory for Human Rights (SOHR).
Speaking about the military situation inside Syria, Moualem said Deir al-Zor, a city and province occupied by Islamic State in the east, was the "fundamental objective" for government forces and was more important to the average Syrian than Idlib, a northwestern province that is an opposition stronghold.