Abandoned tents, foxholes left behind by IS in Syria Abandoned tents, vehicles and foxholes to hide from airstrikes are all that are left from evacuated parts of the Islamic State group's last shred of territory in Syria

AL-OMAR OIL FIELD BASE, Syria -- Abandoned tents, vehicles and foxholes to hide from airstrikes are all that are left from evacuated parts of the Islamic State group's last shred of territory in Syria.

An exclusive video obtained by The Associated Press on Friday showed parts of the tiny pocket of land in the village of Baghouz vacated recently by Islamic State group members and their families. The area has been under attack since September by the U.S.-backed Syrian Democratic Forces who in recent weeks have advanced on the town from three sides, besieging it.

Hundreds of IS fighters, along with thousands of civilians, mostly family members, have evacuated the IS-held area after the SDF alternately applied military pressure and allowed time for evacuees to come out. In the last two weeks, many fighters appeared to be among those evacuating.

But some IS militants are still clinging to a patch of land inside the village and are vowing to fight.

In Washington, a senior defense official estimated that nearly 20,000 people, including 3,500 to 4,000 adult males, have emerged from Baghouz since Feb. 20. The official, who could not be identified by name under Pentagon ground rules, said nearly all of the 20,000, including women and children, are seen as IS followers or adherents.

Baghouz in eastern Syria is the very last speck of territory held by IS, which once occupied a territory the size of Britain over areas straddling both Iraq and Syria that it called a "caliphate." The coalition effort to destroy IS has taken place amid Syria's nearly 8-year-old civil war.

The senior U.S. defense official said the U.S. and the SDF had greatly underestimated not only the number of IS fighters and affiliated civilians holed up in the final slice of IS territory, but also the amount of time it would take to finish off the fight. The official said it would not be a surprise, based on current conditions, if it took another couple of weeks.

The official said the SDF is believed to have about 5,000 IS fighters in captivity, of which about 4,000 are Iraqis and Syrians. The other 1,000 or so are "foreign" fighters from dozens of other countries, the official said.

The video obtained by AP was shot on Wednesday by a fighter from the People's Protection Units, or YPG, the main Kurdish militia which is part of the Syrian Democratic Forces. It showed abandoned tents and vehicles, foxholes in which to hide from airstrikes, weapons and other items left behind by the extremists.

A burned pickup truck and several motorcycles could be seen, as well as scattered pieces of clothing, blankets and cases of ammunition.

Under the cover of heavy coalition bombing on March 1-2, SDF forces advanced on the besieged tent encampment, leaving a corridor for residents to leave. During the airstrikes, an ammunition depot and vehicles packed with weapons were targeted, setting off large explosions and a fire that lasted for at least two days. In the video, fire was still smoldering from some tents, which caught fire in the explosion.

Following that operation, thousands of residents and many fighters evacuated Baghouz over the next four days. But on Friday, only a small group came out, raising speculations that a renewed military offensive was being planned.

Evacuated civilians have described terrible conditions inside the village, with food scarce and people forced to hide underground to escape airstrikes and shelling by the SDF. Many of them were also adamant supporters of the militant group, who defended its tactics.

The evacuees have either been sent to a displaced people's camp to the north or suspected fighters have moved to detention facilities. The U.S.-led coalition takes part in screening and interrogating the evacuees.

The aid group International Rescue Committee said at least 6,000 women and children arrived from Baghouz to al-Hol displaced people's camp on Thursday alone, bringing the total of evacuees over two days to 12,000. The camp, IRC said, has reached "a breaking point," with over 55,000 arriving there since December.

The total population of the camp is now at 65,000 as aid workers are overwhelmed with shortage of tents and resources. At least 100 people, mostly children, died en route to al-Hol or shortly after arriving due to a combination of malnutrition and hypothermia. More than 240 unaccompanied children also arrived at the camp, as well as many with serious injuries.

Separately and on the other side of Syria, Turkey's defense minister said on Friday Turkish and Russian troops will begin patrols of Syria's northwestern Idlib region, where the two countries have created a de-escalation zone.

Hulusi Akar also said that restrictions on Turkey's use of the airspace above the Idlib and Afrin regions have been lifted, signaling the possible use of drones or aircraft to secure those areas.

Russia and Turkey brokered a cease-fire in September for Idlib, the last major stronghold of Syria's rebels. The agreement averted a Syrian government offensive, but has come under strain as al-Qaida-linked militants have seized towns and villages from rival insurgents.

In comments aired live on several Turkish news channels, Akar said Russia would patrol the border outside Idlib while Turkey would patrol inside the de-escalation zone.

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Associated Press National Security writer Robert Burns contributed from Washington.