WASHINGTON — Secretary of State John Kerry said he will move towards a plan B that could involve a partition of Syria if a planned ceasefire due to start in the next few days does not materialize, or if a genuine shift to a transitional government does not take place in the coming months.

“It may be too late to keep it as a whole Syria if we wait much longer,” he told the Senate foreign relations committee on Tuesday.

Kerry did not advocate partition as a solution and refused to specify details of a plan B, such as increased military involvement, beyond insisting it would be wrong to assume that President Obama would not countenance further action.

He also admitted it was possible Russian-backed forces could capture Aleppo, but pointed out that it has been very hard to retain territory in the five-year civil war.

Kerry said he will be meeting with Russia and other world powers in Geneva in the next few days to discuss the modalities of the ceasefire that was agreed on Monday and is due to come into force on Saturday.

Both the Syrian government led by Bashar al-Assad and Syrian opposition body the High Negotiating Council have said they will conditionally abide by the ceasefire, including the identification of territories and groups that would be included in the ceasefire.

The cessation of hostilities, agreed in a phone call between the U.S. and Russian presidents, Obama and Vladimir Putin, specifically excludes ISIS, Nusra Front and other terrorist groups identified by the United Nations security council.

There are serious doubts that the ceasefire will hold or be jointly enforceable by the U.S. and Russia because of complex alliances in the opposition and the way in which terrorist groups are intertwined with legitimate opposition forces.

Kerry suggested partition could form part of an eventual solution, saying “this can get a lot uglier and Russia has to be sitting there evaluating that too. It may be too late to keep it as a whole Syria if it is much longer.” It is the first time Kerry has spoken of partition, although some believe Putin would be content to see this happen.