- ISIS captures Syrian border post, now controls all Syrian stations on border with Iraq
- A Palmyra resident says ISIS fighters went door-to-door in conquered Syrian city
- ISIS lauds fighters for "liberating" Palmyra and taking over a prison and military base
(CNN)"They are everywhere."
That's a 26-year-old Syrian's stark observation about ISIS fighters in Palmyra, detailing the terrorist group's swift, destructive takeover of yet another city in the brutal quest to expand its caliphate in the Middle East.
The capture of Syria's ancient city threatens a UNESCO World Heritage Site described as having "stood at the crossroads of several civilizations," with its art and architecture mixing Greek, Roman and Persian influences, according to that U.N. group.
U.N. and Syrian officials have expressed fears that ISIS will destroy the ruins, just as it flattened the ancient Assyrian city of Nimrud and smashed statues in Iraq's Mosul Museum.
But Palmyra, also known as Tadmur, isn't just a historical site. It's home to tens of thousands of people, many of whom fear they'll meet the same fate as others ISIS has conquered.
They're people like the 26-year-old who asked to remain anonymous for fear of retribution. He's huddled in a house with 50 others, including his family and neighbors who have lost their homes, and he's worried food will run out while his city is under curfew.
After at least 100 Syrian soldiers died in fighting overnight, Syrian warplanes carried out airstrikes Thursday in and around Palmyra, the Syrian Observatory for Human Rights reported. But there's no indication that Syrian ground forces will try to take back the city, 150 miles northeast of Damascus, the capital. Nor that any other countries such as the United States will come to the rescue.
"The world does not care about us," the Palmyra resident said. "All they are interested in is the stones of ancient Palmyra."
'I want to die in my city'
After days of regular sparring, ISIS fighters made a big push early Thursday, and Syrian government forces retreated, according to the Syrian Observatory for Human Rights, which is monitoring the conflict.
"ISIS has taken the prison of Palmyra, the intelligence headquarters, everything," said the executive director of that London-based group, Rami Abdurrahman.
In a daily news bulletin, ISIS credited its fighters with "completely liberating" the city and taking over the prison and a nearby military airbase. It said this assault left "several dead soldiers behind," and it tweeted photos claiming to be from Palmyra that showed bloodied bodies of men who weren't in uniform.
ISIS militants killed at least 17 people in Palmyra, the observatory reported, saying some deaths were beheadings. CNN is not able to confirm this report independently.
The Sunni Muslim jihadists spent much of Thursday implementing a curfew and going door-to-door, apparently looking for Syrian soldiers, according to the 26-year-old. He said eight ISIS fighters went through his house and were "trying to appear friendly."
Civil war broke out four years ago in Syria, providing an opening for groups such as ISIS to emerge and take on forces loyal to President Bashar al-Assad. With its latest offensive, ISIS controls more than half the country -- in parts of 10 of 14 provinces -- as well as "the vast majority of the gas and oil fields," the observatory estimates.
Fueled by its success in Syria, the militant group also has captured swaths of neighboring Iraq, including its second-largest city, Mosul. This week, ISIS seized the key city of Ramadi, a milestone that a U.S. State Department official acknowledged is a major blow in coalition efforts to defeat ISIS.
Thursday it took over the last Syria-Iraq border crossing that was under the control of Syrian troops, the Syrian Observatory for Human Rights said. The Tunef border crossing fell under ISIS control after Syrian regime troops withdrew, the activist group reported. ISIS also controls the Al Waleed border station on the Iraqi side.
But despite ISIS advances, the Palmyra resident who talked to CNN isn't ready to walk away, even to save his life.
"I did not feel safe four years ago," he said. "But (still now), I do not want to leave. I want to die in my city."
Historian: 'Couldn't be higher stakes'
The fall of Palmyra spurred condemnation worldwide, leading to tweets that used #SavePalmyra.
"Am screaming #SavePalmyra out of sheer despair," an activist wrote, "as I dont know who this scream is directed to or what anyone can do."
ISIS has conquered many other parts of Syria and Iraq. But Palmyra stands out for its history.
The city already was a caravan oasis when the Romans overtook it in the middle of the first century. Its importance grew on a trade route linking the Roman Empire to Persia, India and China.
"The ruins are absolutely glorious," said CNN iReport contributor Aradhana Anand, who visited Palmyra in 2010. "(It's) heartbreaking, really."
British historian and novelist Tom Holland has described Palmyra as "an extraordinary fusion of classical and Iranian influences intermixed with various Arab influence as well."
Extensive destruction of Palmyra wouldn't just be a tragedy for Syria. It would be a loss for the world, Holland said.
"Mesopotamia, Iraq, Syria, this is the wellspring of global civilization," he said. "It really couldn't be higher stakes in terms of conservation."
Will centuries-old artifacts be destroyed or sold?
Syria already has seen its storied past damaged and destroyed by war.
Notable casualties include 11th-century Crusader castle Crac des Chevaliers; regime airstrikes severely damaged its walls in 2013. Aleppo's covered market, a formerly thriving part of Syria's economic and social life, was severely damaged in a fire in 2012.
ISIS is "distinctive and horrendous" in how it treats history, Holland said. Syrian antiquities chief Maamoun Abdulkarim said the group poses "the biggest danger" now to his country's artifacts.
"In general, ISIS attacks people first for control," Abdulkarim said. "Second, they attack heritage by destroying for propaganda and ideological reasons. And (third), they work with the mafia to sell the artifacts."
The Syrian government says it has moved many artifacts, including hundreds of statues, to safer locations. But it can't relocate an entire archaeological site.
"We consider this ... a culture battle for humanity and all the world," Abdulkarim said. "Palmyra is very important in the minds of the Syrian people and also the international community. Now we are very afraid."