The United States was not part of the deal by government backers Russian Federation and Iran, and rebel supporter Turkey.
Walid al-Moallem, Syria's foreign minister, explained the decision regarding the recently-brokered deal involvingfour "de-escalation" zones during a press conference in Damascus on Monday.
The general parameters of the so called "de-escalation zones", agreed to by Russia, Turkey and Iran earlier this month are "well understood", Defense Secretary James Mattis told reporters while en route to Copenhagen for a senior-level ministerial with military leaders involved in the anti-Islamic State coalition.
Syrian activists say government forces have taken a village on the edge of a large cease-fire zone encompassing rebel-held territory in northwest Syria.
The Syrian official noted that it was "premature" to talk about whether the agreement would be successful, but noted that the government would "harshly" retaliate against any violation by rebels while the deal was in force.
Washington is not participating in the talks, created to end the conflict between the Syrian regime and rebel groups battling to oust Syrian President Bashar Assad. But Mattis also said the plan poses many unanswered questions, including whether it would be effective.
Mattis said it's not clear yet what effect the zones - in Idlib province, north Homs province, the Ghouta suburbs of Damascus and parts of Syria's southern provinces - could have on the US -led coalition's fight against ISIS. "The devil is always in the details, so we are going to have to look at the details, see if we can work them out, see if we think they are going to be effective", Mattis said.
On another diplomatic track, the United Nations' Syria envoy, Staffan de Mistura, announced plans to reconvene talks in Geneva between representatives of Syrian President Bashar Assad's government and opposition leaders May 16. "I think the global community is united in the sense of wanting to see the ISIS put on its back foot", the defence secretary said.
He said the government aims to regain control of all of Syria, adding that areas bordering Iraq will be a priority.
As the foreign minister spoke to reporters, hundreds of rebels and their families began boarding buses to leave a besieged opposition-held neighborhood of Damascus for rebel-held areas in the country's north, according to state TV and opposition activists.
Top U.S. national security leaders are reportedly weighing the details on a series of proposed safe zones in Syria, in an attempt to stem the violence from the country's six-year civil war.
The evacuation from Damascus' northeastern Barzeh neighborhood is the first such population movement in this area. However, the organization reported that only four buses had left the Damascus suburb of Barzeh, which has been under siege for the past month.
The UK-based monitoring group Syrian Observatory for Human Rights (SOHR) reported continued strikes and fighting across the country including Eastern Ghouta, Aleppo and Idlib.
"All wars eventually come to an end and we've been looking for a long time how to bring this one to an end", Mattis told reporters traveling with him to Copenhagen to meet with USA allies.