Syrians evacuated from the embattled Syrian city of Aleppo during the ceasefire arrive at a refugee camp in Rashidin, near Idlib, Syria, early Monday, Dec. 19, 2016. (AP)

Syrian government forces threatened Tuesday to storm the last opposition enclave in Aleppo, potentially complicating an internationally supported effort to evacuate thousands of civilians still trapped there.

The Lebanese militia Hezbollah, which is fighting in Aleppo in support of Syrian President Bashar al-Assad, said Syrian troops have issued their last call for people to leave the sliver of territory in the city’s eastern districts that is still in the hands of opposition forces.

As many as 25,000 people have been evacuated from the area since Thursday, Robert Mardini, regional director of the International Committee of the Red Cross, said on Twitter.

The evacuation deal — seen as the only way to avoid a bloodbath — marked an effective surrender by rebel fighters in Aleppo; they had controlled the city’s eastern neighborhoods since 2012. Aleppo is a key prize in Syria’s civil war, and the deal represents a major victory for Assad and his allies, including Russia and Iran.

Pro-government activists said many people still in eastern Aleppo are insurgents, although that could not be independently verified.

Evacuations have been taking place since last week as part of an agreement brokered by Russia and Turkey, but disruptions have complicated the effort, and thousands are waiting to be bused to a rebel-held province to the west.

The government on Tuesday authorized an additional 20 U.N. staff members to monitor the evacuations, Jens Laerke, a spokesman for the United Nations, told reporters in Geneva.

The U.N. Security Council voted Monday to deploy international observers to monitor the evacuation routes — a rare international agreement over the fate of Aleppo as government forces close in.

“This will almost triple the number of international staff currently deployed to Aleppo,” Laerke said. He cautioned that monitors still do not have independent access to the evacuation buses.

Pro-government fighters, including militiamen from Hezbollah and Iran, temporarily halted the evacuations to demand parallel pullouts from two nearby Shiite villages under siege by rebels. The United Nations estimates that about 750 people have since been evacuated from the villages, Fouaa and Kefraya, where a rebel blockade has deprived residents of food and medicine.

On Sunday, suspected Islamist rebels burned evacuation buses sent to the villages, temporarily endangering the entire evacuation plan.

Assad’s opponents accuse government forces and allied militiamen of robbing — and even executing — a number of people as they have moved into parts of eastern Aleppo once held by the rebels.

France’s envoy to the Security Council said Monday that he hoped the resolution would prevent Aleppo from becoming another Srebrenica, where thousands of Bosnian men and boys were killed in 1995 as Serb forces seized the town.The text of the resolution allowed five days for the observers to arrive.

The city has been perhaps the most violent flash point of the five-year-old Syrian conflict, which has killed about half a million people and displaced millions.

Aleppo has almost become synonymous with the brutality of Assad’s air raids, which have leveled the city’s eastern neighborhoods. Hospitals, residences and even people gathered on Aleppo’s streets have been routinely targeted by Syrian government and Russian warplanes, as well as by troops and allied militiamen, who have fired artillery and staged ground assaults.

Thousands have waited in the last rebel holdouts as winter temperatures have dipped below freezing. Many have been forced to sleep in bombed-out structures or even on sidewalks.

Zakaria Zakaria in Istanbul contributed to this report.