PENTAGON - A week after U.S.-backed forces announced the start of their final assault on Islamic State territory in Syria, there is little the troops or the countries backing them can be sure of.

Victory over the last remnant of the terror group's self-declared caliphate, still seen as inevitable, has been delayed as a wave of humanity fled the ever-shrinking patch of land IS calls its own.

'We have been consistently wrong'

In fact, just about the only thing anyone is almost sure of is what will not be found when the fighting is finally done.

"We're pretty confident the leadership is not still down in this tiny little, basically, hellhole that remains," a senior U.S. defense official said Friday, referring to Abu Bakr al-Baghdadi, the terror group's self-declared caliph.

As for everything else, "We have been consistently wrong, as have our SDF [Syrian Democratic Forces] partners, on how big this is," the official added, referring to the exodus of IS fighters, women and children from their tiny enclave in the northeastern Syrian town of Baghuz.

Since SDF officials estimated in late February that no more than 1,000 people remained in the bombed-out farming community on the banks of the Euphrates River, the Pentagon estimates about 20,000 people have fled.

Initial estimates from the United Nations and SDF officials suggest the total may be even higher, perhaps closer to 25,000.

Children sit on the back of a truck near the villa Children sit on the back of a truck near the village of Baghuz, Deir el-Zour province, in Syria, March 7, 2019. Children sit on the back of a truck near the village of Baghuz, Deir el-Zour province, in Syria, March 7, 2019.

Pushed to 'breaking point'

"The number of civilians coming out of Baghuz has exceeded any prediction of humanitarian actors," Hedinn Halldorsson, with the U.N.'s Office for the Coordination of Humanitarian Affairs (OCHA) in Damascus, told VOA.

Separately, the International Rescue Committee warned Friday the al-Hol displaced-persons camp was being pushed to a "breaking point" by the exodus from Baghuz, with 12,000 people arriving in just a 48-hour period, pushing the camp's population to more than 65,000.

And more may still be lurking in the kilometers of tunnels and caves extending far beneath the shattered buildings and ragged tents.

A YPG media official tweeted that a message recovered from the phone of an IS fighter claimed 45,000 people had taken refuge in the final corner of the IS caliphate.

From WhatsApp messages of a surrendered Daesh fighter from February 27: “We’re 45.000 people inside [#Baghouz]” — zana amedi (@zana_med) March 8, 2019

Yet as surprising as the numbers have been, U.S. defense officials do not believe it is an accident or a happenstance of the campaign to liberate this part of Syria from IS rule.

"What we are seeing now is not the surrender of ISIS as an organization but a calculated decision," the commander of U.S. forces in the Middle East, Gen. Joseph Votel, told U.S. lawmakers Thursday, using one of many acronyms for the terror group.

A fighter from Syrian Democratic Forces checks a m FILE - A fighter from Syrian Democratic Forces checks a man near the village of Baghuz, Deir, Syria Feb. 22, 2019. FILE - A fighter from Syrian Democratic Forces checks a man near the village of Baghuz, Deir, Syria Feb. 22, 2019.

He and other officials warn IS's goal is to preserve as much of the group's capabilities as possible as it completes a transition from an almost traditional army to a clandestine insurgency, counting on every man, woman and child to do his or her part.

"The vast majority of these are assessed not to be innocent civilians," the senior defense official said of the thousands of stragglers who evacuated Baghuz in recent days.

"Some of these folks have been with ISIS for years and have sort of followed the retreating ISIS army, battle after battle," the official said. "They want to continue this fight even if there isn't a physical area to protect."

More IS Wives Flee Syria's Baghuz Thousands of civilians have evacuated from Baghuz, Syria, in the past two days as U.S.-backed Syrian Democratic Forces (SDF) are trying to recapture the last stronghold of Islamic State (IS) in eastern Syria. More than 6,500 civilians left Baghuz in the last two days alone, SDF officials told VOA Thursday. They said hundreds of IS families, including children and women, were among those fleeing the town. A VOA reporter spoke with some of them as SDF officials were vetting them. SDF said… Thousands of civilians have evacuated from Baghuz, Syria, in the past two days as U.S.-backed Syrian Democratic Forces (SDF) are trying to recapture the last stronghold of Islamic State (IS) in eastern Syria. More than 6,500 civilians left Baghuz in the last two days alone, SDF officials told VOA Thursday. They said hundreds of IS families, including children and women, were among those fleeing the town. A VOA reporter spoke with some of them as SDF officials were vetting them. SDF said…

As if to echo that defiance, an Iraqi woman, who fled Baghuz and identified herself as Oum Mohammed, told cameras with the French news agency AFP this past week, "It's not a defeat, no, it's nothing."

"The brave ones are still left," she added. "Those who remain will win by the grace of God."

While few U.S. or SDF officials believe IS will be able to prevail in some sort of last stand at Baghuz, many worry that over the long term, the terror group could find a way to re-emerge and perhaps even attempt, at some point, to reclaim territory.

Women hug near the village of Baghuz, Deir Al Zor Women hug near the village of Baghuz, Deir el-Zour province, in Syria, March 7, 2019. Women hug near the village of Baghuz, Deir el-Zour province, in Syria, March 7, 2019.

Sleeper cells a concern

U.S. defense and intelligence officials estimate IS has tens of thousands of fighters positioned across Syria and Iraq, some as part of sleeper cells that have already begun to activate.

Kurdish officials in Iraq warn that in some parts of Iraq, the terror group has free rein, essentially ruling the night even as government troops and police seem to be in charge by day.

The group's financial networks, while significantly degraded, remain functional.

And whether at the SDF screening centers outside Baghuz or in displaced-persons camps, their fervor has not abated.

In one incident at al-Hol this past Tuesday, security guards were forced to fire gunshots to disperse a crowd of about 200 angry women who were demanding to know what had become of their male relatives.

Fighters from the Syrian Democratic Forces stand n Fighters from the Syrian Democratic Forces stand near a group of men, near Baghuz, Deir el-Zour province, Syria, March 6, 2019. Fighters from the Syrian Democratic Forces stand near a group of men, near Baghuz, Deir el-Zour province, Syria, March 6, 2019.

Cooks and janitors?

For now, most of the men are being held in SDF detention facilities, which are being pushed to the limit as the number of those in custody has about doubled in recent weeks.

Many of the men claim to be cooks or janitors, but U.S. officials say almost all 5,000 or so are in fact IS fighters.

More than 1,000 are foreign fighters, hailing from 50 countries spanning Europe, Central Asia, North Africa and the Middle East. The remainder come from Syria or Iraq.

U.S. officials suspect many of them are content to bide their time in prisons, waiting for a chance to escape and eventually rejoin the struggle.

The U.S. believes the SDF can handle them, for now, but that without more aid from the international community, another crisis may erupt.

"This is a serious generational problem that if not handled properly will sow the seeds of future violent extremism," Votel said.

A boy looks at the camera near Baghuz, Deir Al Zor A boy looks at the camera near Baghuz, Deir el-Zour province, Syria, March 5, 2019. A boy looks at the camera near Baghuz, Deir el-Zour province, Syria, March 5, 2019.

Waiting game

For now, though, the focus remains on Baghuz, where at least on Friday, the exodus has stopped.

"We are waiting for [Saturday] morning or perhaps until the afternoon," SDF spokesman Mustafa Bali told Reuters Friday. "If no civilian or terrorist comes out, we will launch our military operation anew."

U.S. officials, though, remain cautious, warning IS may simply be trying to buy more time.

"They'll negotiate and fight, and negotiate and fight," the senior defense official said late Friday.

"Maybe we're done in the next couple of days," the official said. "I hesitate to put a number on it."

VOA's Margaret Besheer contributed this report.