The ruins of an ancient city that have withstood centuries of conflict in the Syrian desert are now facing their greatest threat yet: the militants of the Islamic State.

Activists, officials and citizens of the city say ISIS has launched a prolonged assault on Palmyra, an "oasis in the desert" north of Damascus that the U.N. cultural agency UNESCO says contains the "monumental ruins of a great city that was one of the most important cultural centers of the ancient world."

See also: Satellite images show historic Syrian sites reduced to rubble

The group's Director-General, Irina Bokova, has now appealed to the world for help saving the site it describes as having "outstanding universal value."

Camels go past stone pillars, a relic of the Roman Empire, in the desert city of Palmyra, central Syria, on Oct. 1, 2010.

"We must save Palmyra," said UNESCO Director-General Irina Bokova on Thursday as word of the militants advance on the city spread. "The site has already suffered four years of conflict, it suffered from looting and represents an irreplaceable treasure for the Syrian people and for the world. I appeal to all parties to protect Palmyra and make every effort to prevent its destruction."

Ruins in the Syrian desert in the ancient city of Palmyra, Syria, 03 June 2009. Image: Chris Melzer/picture-alliance/dpa/AP Images/Associated Press

On Friday, Bokova turned up the heat.

"The stakes are high," declared Bokova at a conference in Egypt on the theft and destruction of antiquities in the region. "The destruction and looting of archaeological sites and museums have reached unprecedented levels. The destruction of cultural heritage, the cultural cleansing, is being used as a tactic of war to terrify populations, to finance criminal activities and to spread hatred."

"We must consider it for what it is: A war crime."

A video posted on internet on February 26, 2015 shows ISIS or Daesh (Daech) or "Islamic State" group militants destroying statues inside the Nineveh museum, northern Iraq.

A video posted on internet on February 26, 2015 shows ISIS or Daesh (Daech) or "Islamic State" group militants destroying statues inside the Nineveh museum, northern Iraq. Some of the statues date from 8th century BC. Image: Balkis Press/Sipa USA/Associated Press

On Twitter, historians lamented what appears to be Palmyra's seemingly impending destruction. One said it would "be like the felling of the grandest tree in the forest of civilization."

Palmyra is where it all ends. It'd be like the felling of the grandest tree in the forest of civilisation. — Tim Stanley (@timothy_stanley) May 15, 2015

Not everyone's convinced ISIS is after the city's historic ruins, though. Some even say that the warnings over Palmyra's imminent destruction may only serve to hasten its demise.

Charles Lister, a fellow at the Brookings Doha Center, said on Twitter it wasn't the ruins the militants were after — just any "idols" that still stand in their midst. "ISIS only destroys objects seen as 'idols'," said Lister. "Palmyra ruins are not that. Global focus & fear over their fate only encourages destruction."

#ISIS only destroys objects seen as "idols" - Palmyra ruins are not that. Global focus & fear over their fate only encourages destruction. — Charles Lister (@Charles_Lister) May 15, 2015

"Why would ISIS destroy Palmyra ruins?" asked Hassan Hassan, co-author of the New York Times bestseller ISIS: Inside the Army of Terror. They "might destroy the mini-museum nearby but not the columns. If anything the warnings will make it do that...to annoy and make headlines," he added.

Why wud Isis destroy Palmyra ruins? Might destroy the mini-museum nearby but not the columns. If anything the warnings will make it do that — Hassan Hassan (@hxhassan) May 15, 2015

The U.N.'s cultural agency says the city stands as a "unique relic" of the first century and "a masterpiece of architecture and Roman urbanism." It's impacted centuries of architecture and attracted thousands of tourists over the years.

The site was inscribed as a World Heritage Property in 1980 and added to the UNESCO List of World Heritage in Danger in 2013. It's already suffered moderate-to-severe damage in the fighting, according to a recent damage assessment by the United Nations Institute for Training and Research (UNITAR).

A 2014 damage assessment by United Nations Institute for Training and Research (UNITAR) described the extend of destruction to the historical sites in Palmyra. Image: United Nations Institute for Training and Research (UNITAR)

While the ruins still stand, time is running out, local officials say.

The world "must mobilize before, not after, the destruction of the artifacts" at Palmrya, Syria's director of antiquities & museums told the AFP. "If ISIS enters Palmyra, it will be destroyed and it will be an international catastrophe," Mamoun Abdulkarim said of the city's temples, ruins and tombs. "You can hide the objects, but how can you save ancient architecture?"

The fight for the city

Activist groups such as the Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees said that Syrian government warplanes have been attacking positions of the Islamic State group on the eastern edge of Palmyra. There has also been fighting on the ground, the groups said.

A photo posted on internet on April 7, 2015 shows ISIS or Daesh (Daech) or "Islamic State" group militants posing in Yarmouk (Yarmuk) Palestinian camp, located in a suburb of Damascus, Syria, that is partially now under their control. Image: Balkis Press/Sipa USA/Associated Press

On Friday, Syrian state news agency SANA said troops were "chasing" IS fighters in several areas north and east of Palmyra.

A local fighter there told Syria Direct the city's streets are nearly empty of pedestrians, with snipers targeting residents who try to flee.

"We are afraid. It's a real war. We didn’t sleep at all last night. We can't go out of the house and we don't know what's going to happen," a resident of the town told Middle East Eye. "There are snipers outside. The war is on."

Another asked how he could care about the ruins when lives are at risk. "I am so anxious. I don't care who controls it, we just don't want a long battle," he said of the impending fight. "There are people who care about the ruins. But [how can] you care about the ruins? There are civilians here."

More than 200,000 civilians have been reported killed in Syria's civil war and dozens of historic monuments, mosques, churches and ruins have been destroyed — by both sides of the conflict.

Some information in this report was provided by the Associated Press.