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Syrian regime military helicopters on Thursday dropped leaflets over parts of the rebel-held northwestern province of Idlib, calling on residents to reconcile with Assad's authorities as warplanes simultaneously pounded the region, opposition activists said.

The message came as a top humanitarian adviser to the UN warned that "war cannot be allowed to go to Idlib."

Jan Egeland said the UN had appealed to Turkey to open its border to refugees from Idlib should the Syrian government decide to attack the province, now the last major bastion of the armed opposition in the country, and home to over one million internally displaced Syrians.

Humanitarian organisations have shared the GPS coordinates of 235 sites, including the locations of medical facilities and schools, with the Russian, Turkish and US militaries, in the hopes that warring parties would avoid targeting them in the eventuality of a battle, said Egeland.

Read more: Selling a phoney peace in Syria

Egeland said a push by the regime would destroy the province and aggravate an already dire humanitarian situation marked by insufficient shelter, and substandard hygiene, water and medical distribution. Some 2.9 million people are now residing in the province, said Egeland.

But the predominant strategy of Bashar al-Assad's forces has been to target precisely those institutions where medical professionals work and civilians shelter, according to rights groups that have been following the seven-year-long conflict.

"We must learn from eastern Ghouta, Aleppo, Homs, Raqqa, and elsewhere: It is no way to liberate an area by leveling everything to the ground," Egeland said, citing previous offensives by Assad's forces that crushed rebels and displaced millions of Syrians in major cities in the country.

The government's leaflets promised the war "is close to an end" and called for Idlib residents to join in reconciliation "as our people did in other parts of Syria," according to photos posted by the Britain-based Syrian Observatory for Human Rights conflict monitor.

The leaflets signed by the Armed Forces Command had photos of Syria before and after the war with a caption that read: "This is how Syria was before terrorism."

Ibaa news agency, the media arm of al-Qaeda-linked Levant Liberation Committee, said the leaflets were dropped over the towns of Taftanaz, Kfarya and Binnish adding the activists in the area poked fun at the leaflets.

Activists said even as helicopters dropped the leaflets, warplanes pounded several rebel-held areas elsewhere in Idlib, which has become home to tens of thousands of internally displaced people.

Idlib is the last major rebel-held region in Syria and there are concerns that a government offensive would displace hundreds of thousands of people inside the province bordering Turkey.

Ibaa said the leaflets were part of a "psychological war" by the government that has not been able to capture Idlib militarily.

In southern Syria, a woman who was among dozens of women and children kidnapped by the Islamic State group in the southern province of Sweida last month died while in captivity, activists said.

Some 30 women and children have been held since the July 25 attack by IS on villages in Sweida and a nearby village that left more than 200 people dead. Since then, negotiations have been ongoing to exchange the hostages for IS fighters.

Read more: Who's who in Syria?

Last week, IS killed one of the hostages, a 19-year-old boy, to pressure the government and local officials in the negotiations.

The Sweida 24 activist collective posted a photograph of the woman while in detention and another after her death. It said the photo of the woman was sent by IS saying it was a natural death due to "harsh health conditions."

Syria's civil war has cost the country $388 billion in economic damage, the UN's Economic and Social Commission for Western Asia agency said in a new estimate published Thursday.

Monitoring groups say at least 400,000 people have been killed in Syria's civil war. More than 11 million others — or half of Syria's pre-war population — have been displaced from their homes, according to the UN, including more than 5 million who have been made refugees abroad, since war broke out in 2011.

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