"I don't know whether or not Syria can be put back together again," Brennan said on Friday at the annual Aspen Security Forum in Colorado.
Brennan went on to add that "there’s been so much blood spilled, I don’t know if we’re going to be able to get back to [a unified Syria] in my life time."
John Kerry, US Secretary of State, expressed similar fears back in February, stating that the coalition was moving towards "Plan B," implying a potential partitioning of Syria if a meaningful ceasefire could not materialize.
"It may be too late to keep it as a whole Syria if we wait much longer," Kerry told the US Senate foreign relations committee.
Staffan de Mistura, UN envoy to Syria, also acknowledged that the possibility of a federal division of the country had not yet been ruled out.
He told Al Jazeera in March, "All Syrians have rejected the division [of Syria] and federalism can be discussed at the negotiations."
US President Barack Obama has been accused of adopting a more disengaged approach to Syria. In an interview with the BBC in April, Obama said all parties needed to “sit down at the table and try to broker a transition.” Adding, “That’s difficult.”
He described Syria as a “transnational” issue that needed a similar response, saying that without international cooperation, “we are far weaker and we won’t solve these problems.”
Brett McGurk, Special Presidential Envoy for Global Coalition to Counter ISIL, also speaking at the conference in Aspen, accused Russia of being either “unable or unwilling to deliver on the removal of Assad.”
Up to date, 400,000 people have been killed due to the ongoing civil according to UN estimates, with driven about 11 million people from their homes, a figure that leaves officials on all fronts disillusioned over the prospect of reunification.
While the Syrian opposition seek to overthrow the government, President Bashar al-Assad has pledged to "liberate every inch" of the country lost to rebel forces.
Riad Hijab, coordinator for the opposition's High Negotiations Committee (HNC) at the Geneva peace talks, said that "any mention of federalism or something which might present a direction for dividing Syria is not acceptable at all," when the idea was first brought up in March.
In contrast, the Syrian Kurdish PYD party, which has wide influence over the country's Kurdish areas, and several allied groups have announced plans to create an autonomous federation in the northeast of the country, including the three Kurdish cantons of Jazira, Kobani and Afrin.
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