The implementation of a deal to evacuate four besieged Syrian towns has been delayed over opposition from residents and last-minute negotiations, a monitor said Tuesday.

The evacuation of more than 30,000 people from the towns of Fuaa, Kafraya, Madaya and Zabadani, brokered by rebel supporter Qatar and regime ally Iran, was scheduled to start Tuesday.

But the Britain-based Syrian Observatory for Human Rights monitor said it had been delayed, likely to Thursday or Sunday.

Fuaa and Kafraya are government-held Shiite-majority villages in the otherwise rebel-held province of Idlib, while Madaya and Zabadani are opposition enclaves surrounded by regime forces in Damascus province.

Observatory chief Rami Abdel Rahman told AFP that residents in all four towns had expressed reservations about the evacuations.

He said negotiations were also ongoing about the status of Madaya and Zabadani after the evacuation.

"The issue is whether the civilians will leave, because some want to stay, and whether the regime forces will enter Madaya and Zabadani in full, or there will be an agreement just to raise the flag," he said.

All 16,000 residents of Fuaa and Kafraya are expected to leave under the deal, which prompted resistance from locals, the Observatory said.

Syria's government has negotiated a series of "reconciliation deals" with formerly rebel-held towns, offering fighters safe passage in exchange for surrender.

In some cases army troops enter the areas after the deal is implemented, but in others the government flag has simply been raised and administrative control resumed.

The four towns are part of an existing deal reached in 2015 that has seen simultaneous evacuations and aid deliveries, the last of which took place in November.

But the aid has proved insufficient, with reports of malnutrition and deaths from lack of food and medical care in the towns.

At least 600,000 people are living under siege in Syria, according to the United Nations, with another four million people in so-called "hard-to-reach" areas.

More than 320,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.