GENEVA (Reuters) - Syria is heading for a catastrophic partition and could see the return of Islamic State if there is no inclusive peace settlement, U.N. Syria envoy Staffan de Mistura said on Monday.

A Kurdish elderly man walks with a cane at a damaged site in Afrin, Syria March 19, 2018. REUTERS/Khalil Ashawi

“The truth is that a soft, long-term partition of Syria, which (is) the one that we are witnessing at the moment, in different areas of control, will be a catastrophe, not only for Syria but for the whole region,” he told an audience at Geneva’s Graduate Institute.

“Without an inclusive political process, including those who are excluded, particularly the majority, the Sunnis, Daesh will come back,” he said, referring to Islamic State by its pejorative Arabic name.

De Mistura, holding up a map of Syria with different colours for territory held by different parties, said: “This is fragmentation, this is in fact a country which has areas under the influence of other countries.”

“This is not sustainable, he added. “I believe at end of day Syria has to remain unified.”

He said he believed neither the European Union nor the World Bank would help fund the estimated $352 billion cost of rebuilding Syria unless there was a political process with a new constitution, elections under U.N. supervision and power-sharing.

Without that any military victory would be a pyrrhic victory, he said.

No country actually wanted to see Syria break up, and Russia and the United States had a common interest in seeing Islamic State defeated and were talking to each other, which was positive, de Mistura said.

“Much of what we are seeing in Iraq, in Syria, in Lebanon, in Yemen is connectable to the tectonic fight between one country and another: Iran and Saudi Arabia, Shia and Sunni. We cannot hide it,” he said.

An agreement between those two countries to sit down and talk would have an immediate impact across the region.

But he saw more potential for creative diplomacy between the United States and Russia, he said.

But he saw no chance of “diplomacy backed by force”, a formula used to end the war in former Yugoslavia in the 1990s.

“I think there is no willingness by any country to get involved militarily inside Syria in a pro-active way, except Russia which is there to defend the government, and others there for other reasons but not necessarily to find a solution.”