"ISIS has taken the prison in Palmyra, the intelligence HQ, everything ... everything," executive director Rami Abdurrahman told CNN by phone.
The Sunni jihadists stormed Tadmur, a modern city just a few hundred meters from the ruined temples and colonnades of Palmyra on Wednesday and advanced early Thursday.
At least 100 regime troops were killed overnight fighting ISIS, according to the Syrian Observatory for Human Rights.
ISIS militants were going door-to-door looking for Syrian regime soldiers Thursday, one Palmyra resident who asked to remain anonymous for fear of retribution told CNN.
"They (ISIS) are everywhere," another resident told CNN, adding that militants have put the city on curfew.
The takeover by ISIS militants can only add to fears that the group will destroy Palmyra, just as it bulldozed
the ancient Assyrian city of Nimrud and took sledgehammers
to statues in Mosul Museum.
Syrian director of antiquities Maamoun Abdul Karim told AFP
last week: "If ISIS enters Palmyra, it will spell its destruction."
UNESCO Director-General Irina Bokova has also called for the protection of the site.
"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," she said in a statement.
Why is Palmyra so precious?
Palmyra, also known as the "bride of the desert," is an exquisite collection of ruins in the desert northeast of Damascus.
Situated in an oasis, Palmyra was once a monumental city sitting on an important trade route linking Persia, India and China with the Roman Empire.
Its history as an important caravan city at the crossroads of ancient civilizations is reflected in the eclectic mix of architectural styles found among its colonnades and temples.
British historian and novelist Tom Holland describes the site as "an extraordinary fusion of classical and Iranian influences intermixed with various Arab influence as well."
Destruction of Palmyra wouldn't just be a tragedy for Syria, it would be a loss for the entire world, he adds.
"This isn't just about Middle Eastern history, these are the wellsprings of the entire global culture. Mesopotamia, Iraq, Syria, this is the wellspring of global civilization. It really couldn't be higher stakes in terms of conservation."
Is Palmyra useful to ISIS in other ways?
Prof. Fawas Gerges of the London School of Economics, whose forthcoming book, "ISIS: A Short History
," looks at the terror group, says that it takes opportunities when it can.
"It is looking for the soft bellies, the weakest spots ... and the Syrian army didn't have any major troops there."
"Of course, capturing cities and towns provides them with resources. It's the war economy," he said.
According to nonprofit news organization Syria Direct, the Syrian regime considers the modern town at the site "the first line of defense" against ISIS attacks from the eastern regions.
They also report that the town is a point of defense for surrounding oil and gas fields, including the Shaer gas fields
, important for Syria's electricity sector.
Centrally located, control of this area would also give ISIS a clean line down the highway to Damascus and up to Homs.
Why does ISIS want to destroy Palmyra?
This is not the first time ancient sites have been destroyed during Syria's brutal four-year civil war.
Notable casualties include 11th century crusader castle Crac des Chevaliers; its walls were severely damaged
by regime airstrikes in 2013.
Aleppo Souk, a formerly thriving part of Syria's economic and social life, was severely damaged in a fire in 2012.
"What is distinctive and horrendous about IS' mode of operation is that they are deliberately going out of their way to destroy (ancient artifacts)," says Holland.
ISIS propaganda claims the Islamist militants are destroying idols or false gods and following in the footsteps of the Prophet Mohammed, who smashed statues in Mecca.
The terror group is media savvy and knows that bulldozing ancient sites will make the West sit up and take notice.
"They depend on the sugar rush that comes from staging atrocities and they know that one way to get Western attention ... is to destroy ruins that have a particular place in the heart of the entire world," says Holland.
Hypocritically, ISIS sells the artifacts for huge profits.
"They have networks that allow them to traffic in cultural treasures," says Gerges. "They have made tens of millions of dollars selling artworks."
So many people are dying in Syria, do buildings really matter?
It's not a separate issue, argues Holland.
"If you think about the Nazis, they destroyed synagogues as well as the people who worshiped in them," he says, adding that Palmyra was a melting pot where different cultures met and fused.
"So, it's the perfect embodiment of what Syria was before this devastating civil war began."
Gerges said that ISIS is trying to culturally cleanse the land -- both diversity among people and cultural diversity.
Diversity goes against ISIS' very ethos and foundation.
"They want to cleanse the land, literally, whether it's Christians, Kurds, Yazidis."
Or the ancient treasures built by them.