Some 20,000 people who have fled Baghuz in recent weeks have been taken to a camp
The UN has expressed concern about the fate of some 200 families reportedly trapped in the last tiny area of Syria still held by the Islamic State group.
Human rights chief Michelle Bachelet said the many women and children in Baghuz were apparently being actively prevented from leaving by IS militants.
They also continued to be subjected to intense bombardment by US-led coalition forces and allied Syrian fighters.
Ms Bachelet demanded that safe passage be provided to those wanting to flee.
“Those wish to remain must also be protected as much as possible,” she added. “They should not be sacrificed to ideology on the one hand, or military expediency on the other.
“If protecting civilian lives means taking a few more days to capture the last fraction of land controlled by [IS], then so be it.”
IS militants are reportedly confined to tents pitched on top of a network of tunnels and caves
Five years ago, IS controlled 88,000 sq km (34,000 sq miles) of territory stretching from western Syria to eastern Iraq, proclaimed the creation of a “caliphate”, imposed its brutal rule on almost eight million people and generated billions of dollars from oil, extortion, robbery and kidnapping.
Now, an estimated 300 militants and hundreds of civilians are surrounded inside about 0.5 sq km (0.2 square miles) of land in the Baghuz area, which is in the Middle Euphrates River Valley near the border with Iraq.
Syrian Kurdish and Arab fighters from the US-backed Syrian Democratic Forces (SDF) alliance launched an offensive to capture Baghuz on 9 February.
On Monday, unconfirmed reports suggested that the IS militants were seeking to negotiate safe passage to the opposition-held north-western province of Idlib.
There was no confirmation from the SDF. But a spokesman appeared to dismiss such an idea on Tuesday, insisting the militants had “only two options – either they surrender or they will be killed in battle”.
“We are working on secluding and evacuating civilians and then we will attack. This could happen soon,” Mustafa Bali was quoted as saying by AFP news agency.
US-backed SDF fighters launched an assault on Baghuz this month
Although no civilians have reportedly made it out of Baghuz in the past three days, some 20,000 have been taken by the SDF to a makeshift camp for displaced people at al-Hol, in Hassakeh province, in recent weeks.
Among them are the wives and children of IS militants and many foreign nationals, including the British teenager Shamima Begum, who was 15 when she ran away from her home to join IS four years ago.
The International Rescue Committee said on Monday that at least 62 people had died on their way to al-Hol, two thirds of them children under the age of one. Exhaustion and malnutrition were the principle causes of the deaths.
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Ms Bachelet also said she was alarmed by an upsurge in attacks and civilian casualties in Idlib province, where a takeover by a jihadist group linked to al-Qaeda, Hayat Tahrir al-Sham, has jeopardised a truce brokered by Turkey and Russia in September.
The Syrian government’s bombardment of a demilitarised buffer zone, which runs along the frontline in Idlib and areas of northern Hama and western Aleppo provinces, started to escalate in December and has further intensified in recent days, according to the UN.
At the same time, there has been an increase in fighting among rebel and jihadist factions, and also in the use of improvised explosive devices in areas they control.
On Monday, at least 16 civilians, including women and children, were reportedly killed by two bomb explosions in the Qusour district of Idlib city. The second blast appeared to have been designed to kill people, including medical workers, trying to come to the aid victims of the first.
Another nine civilians, including four women and two boys, were meanwhile reportedly killed by government strikes on Khan Sheikhoun on Friday and Saturday.
How the jihadist group rose and fellOctober 2006
The jihadist group announces the creation of the Islamic State of Iraq (ISI) and in April 2010, Abu Bakr al-Baghdadi becomes its leader.
The Islamic State of Iraq (ISI) begins seizing control of territory in Syria, including the city of Raqqa. In April that year, al-Baghdadi changes his group’s name to Islamic State in Iraq and the Levant (Isil or Isis).
Isis conquers over a dozen Iraqi cities and towns like Mosul and Tikrit, and seizes Syria’s largest oilfield in the Homs province. On 29 June, the jihadist group formally declares the creation of a caliphate and becomes known as Islamic State (IS).
IS fighters begin killing and enslaving thousands from the Yazidi religious minority in northern Iraq and release the first of several videos of Western hostages – journalists and aid workers – being beheaded.
The US begins air strikes, starting with attacks on the de-facto IS capital of Raqqa.
IS is at the height of its control, ruling over almost eight million people across 88,000 sq km (34,000 sq miles) from western Syria to eastern Iraq. It is also generating billions of dollars from oil, extortion, robbery and kidnapping.
The Syrian government recaptures the ancient city of Palmyra, but loses it again in December 2016 and then finally recaptures the destroyed Unesco World Heritage site in March 2017.
Iraqi forces liberate Mosul, but the 10-month battle leaves thousands of civilians dead, more than 800,000 displaced and much of Iraq’s second city destroyed.
IS loses control of Raqqa to the Kurdish-led Syrian Democratic Forces (SDF) alliance, ending three years of rule.
Iraq’s government declares victory over IS after retaking full control of the Iraqi-Syrian border.
US President Donald Trump says the jihadist group is close to being defeated, after a battle for the final IS-held territory on the Syrian-Iraqi border lasted weeks.