The head of the US Central Intelligence Agency has said he is not optimistic about the future of Syria remaining one country.

John Brennan's comments are a rare public acknowledgement by a senior US official that Syria may not survive a five-year civil war in its current state.

"I don't know whether or not Syria can be put back together again," he said on Friday at the annual Aspen Security Forum in Colorado.

"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 lifetime."

John Kerry, US secretary of state, expressed similar fears in February, saying he would move towards a "Plan B" that could involve a partition of Syria if a ceasefire did not materialise in advance of peace talks in March.

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"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.

However, he did not directly advocate for partition as a solution.

Several weeks later, Staffan de Mistura, UN envoy to Syria, said the possibility of a federal division of the country had not been taken off the table.

At the time, major powers close to the UN-brokered talks discussed a potential federal break-up of the country, which would grant broad autonomy to regional authorities, while maintaining the country's unity as a single state.

"All Syrians have rejected the division [of Syria] and federalism can be discussed at the negotiations," De Mistura told Al Jazeera in March.

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President Bashar al-Assad pledged in June to "liberate every inch" of the country lost to rebel forces.

After five years of war that have left nearly 400,000 people dead, according to UN estimates, and driven about 11 million people from their homes, Syrian territory has been carved up and divided between the government and its allies, Kurdish fighters, various opposition groups and the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group.

The Syrian opposition has categorically rejected the idea of federalism.

"Any mention of federalism or something which might present a direction for dividing Syria is not acceptable at all," Riad Hijab, coordinator for the opposition's High Negotiations Committee (HNC), said when the idea was proposed in March.

In contrast, the Syrian Kurdish PYD party, which has wide influence over the country's Kurdish areas, and several allied groups announced in March plans to create an autonomous federation in the northeast.

The autonomous region, known as Rojava, includes Jazira, Kobani and Afrin - three distinct enclaves, or cantons, under Kurdish control.