A ceasefire between Islamists fighting in the north of Syria and President Bashar al-Assad’s allies raises the specter of potentially deadly population transfers five years into a civil war that has divided the Levant country along sectarian lines.

The New York Times reports that under a truce struck between opponents and supporters of Assad’s regime, Shia Muslims in the northern Idlib Province would be moved to regime-held areas of Syria while Sunni rebels and their families from Zabadani, a hill station on the Lebanese border, would be allowed to move to Idlib instead.

Idlib, which borders Turkey and overlooks the coastal heartland of Assad’s Alawite tribe, fell into rebel hands earlier this month following a two-year siege.

Zabadani commands the main highway from Damascus to Beirut, Lebanon’s capital. Hezbollah, the Shia militant group that fights on Assad’s side, operates out of Lebanon.

If the deal holds, it would be the most far-reaching since a pact struck more than a year ago that allowed the evacuation of rebel fighters from the center of Homs, Syria’s third largest city, according to The New York Times.

But it would also be another step toward the formal partition of Syria along sectarian lines with the risk of worse conflagration.

The agreement, which would also cease attacks on civilians in both areas, was negotiated between an Islamist rebel group fighting in the north and Iran, Assad’s ally. Qatar and Turkey, which both support the largely Sunni opposition in Syria, reportedly brokered the truce.

The self-declared Islamic State, which controls large parts of eastern Syria and battles both Assad’s forces and other opposition groups, was not involved.

As Russia — another ally of Assad’s — moves fighter jets, tanks and troops into Latakia to reinforce loyalist positions there, it is starting to look like the regime is giving up on ever recapturing the whole of Syria and instead hunkering down in a strip of territory that runs from Latakia in the north via Homs and Damascus to the Jordanian border. The Atlantic Sentinel reported as early as 2012 that this might be Assad’s fallback strategy. It would give him control over the areas populated by Syria’s Alawites, Christians and Druze and leave the Sunni-majority east to what is now the Islamic State.

But the remnant state would still have hundreds of thousands if not millions of Sunni residents. There are no reliable statistics on ethnicity and religion since the Syrian census stopped asking about faith in 1960.

Reports surfaced early in the war of Alawite squads cleansing Sunni towns in the Nusayriyah Mountains that shield Latakia from the rest of the country. The Times of Israel, basing itself on stories in Arab media, reported last month that forces from Iran, the Middle East’s largest Shia states, were helping raze homes in Damascus to force Sunnis out.