Syria is home to six World Heritage Sites, places frozen in time with some structures that pre-date the Crusades. The six sites have been singled out and selected by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as being "of special cultural or physical significance."

But years of war and strife have left their mark on the country's ancient landmarks, even before American airstrikes began this week.

A new report from the American Association for the Advancement of Science shows the extent of that destruction: Using high-resolution satellite imagery, AAAS discovered that five of Syria's six World Heritage Sites had sustained significant damage.

Only the ancient city of Damascus appears to have been spared.

The U.S. missile strikes that began Monday night focused on northern and eastern Syria and included targets around the old city of Aleppo, which is a UNESCO-designated World Heritage Site, one of about 1,000 around the globe.

There has been concern over Syria's World Heritage Sites since the beginning of the conflict -- and in 2013, the World Heritage Committee placed all six Syrian sites on its "danger list" of places at risk of being irreparably damaged or lost forever. Reports have even emerged of fighters-turned-archaeologists plundering the country's -- and civilization's -- historical treasures to finance fighting.

"Antiquities officials in Iraq and Syria warn of a disaster as the region’s history is erased," the Associated Press reported.

The AAAS report zeroed in on the UNESCO World Heritage Sites in Syria, using satellite images to create before-and-after comparisons. Here are their findings.

Ancient city of Aleppo

Added to the World Heritage list in 1986, Aleppo was at the crossroads of trade routes from the 2nd millennium B.C. and includes multiple structures of significance: a 13th Century citadel, the 12th Century Great Mosque and palaces, caravanserais, madrasas and hammams from the 17th Century.

You can click here to zoom on on the damage to Aleppo. Here is a before-and-after comparison showing damage to the Great Mosque, Suq al-Madina.

In the image above, a green arrow points to damage on the roof of the Suq al-Madina. A red arrow points to the minaret of the Great Mosque, which has been destroyed. Blue arrows point out two craters on the eastern wall. Yellow arrows point to multiple nearby structures that were heavily damaged.

The area south of the citadel in Aleppo is home to numerous government buildings, where heavy damage occurred during the fighting.

In the image above, a red arrow points to the Ministry of Justice building, which was heavily damaged. The green arrow points to heavy damage on the Khusriwiye Mosque. A blue arrow points to the spot where the Carlton Citadel Hotel once stood -- it was completely destroyed. Yellow arrows point to surrounding historical structures that were also destroyed.

Just one month later, the Khusriwiye Mosque had been almost completely demolished, as shown by the green arrow in the image above. An orange arrow points to heavy damage to the Grand Serail. A purple arrow points to  the Hammam Yalbougha an-Nasry's dome, which was destroyed.

Elsewhere in Aleppo, the Khan Qurt Bey caravanserai, which dates back to the 15th Century, sustained heavy damage to its east wall, while most structures directly to the east of the site were completely destroyed.

In the image above, a blue arrow points to the east wall of the Kahn Qurt Bey caravanserai, with yellow arrows pointing at nearby structures that were heavily damaged.

Ancient city of Bosra

This city served as the capital of Arabia, a Roman province, and is home to ruins from the Roman, Byzantine and Islamic civilizations. There is a 2nd Century Roman theater within the city, as well as Al-Omari Mosque, one of the oldest-surviving mosques.

Reports of shelling and bombs have emerged from the area during the Syrian conflict. Bosra was added to the UNESCO list in 1980.

There was little damage shown directly to the Roman theater, although war has been present; snipers have been shown shooting from the theater and fortress.

Yellow arrows show how ramps and berms have been constructed near the theater’s east entrance, and a small hill at the site to the west has been partially excavated.

These next images show the Al-Omari Mosque.

Yellow arrows point to spots that are likely shell craters, which experts say caused structural damage throughout the site, including a hole in the mosque's roof. The standing ancient Roman ruins also appear to have a shell crater.

Ancient site of Palmyra

This site served as an oasis and was one of the most important cultural centers of the ancient world. Added to the UNESCO list in 1980, Palmyra is home to Greco-Roman and Persian ruins, including ancient temples, a theater and a Roman-period street.

Palmyra was a top tourist attraction before the conflict, but looters, military occupation and firefights have plagued the area since.

The image above shows how the park been disrupted by the construction of a new road traversing the site. Pink areas point to earthen berms, which are being used to provide cover for military vehicles.

Elsewhere, trailers have been put into place at the Fakhr-al-Din al-Ma'ani Castle.

The road leading to the castle has been reinforced with earthen berms, shown in the image above. Two trailers have been put into place near the citadel, which is also shown from ground-based photographs.

The ancient city wall is indicated by a yellow arrow in the image above. A purple arrow points to the barracks of Diocletian’s camp.

In the image above, a pink arrow points to a portion of the wall that was destroyed to make way for a military encampment. Blue arrows point to where the wall was fortified by earthen berms. The terrain in the camp has also been softened.

Ancient villages of Northern Syria

Also known as the "Dead Cities," this site is made up numerous villages that date back to the 1st and 7th centuries and were abandoned between the 8th and 10th centuries. The villages are grouped together in eight parks with ruins that represent the shift from paganism to Byzantine Christianity.

These villages, which include stunning remains of dwellings, pagan temples, churches and bathhouses, were added to the World Heritage list in 2011.

Not all of the parks were included in the AAAS report, due to a lack of satellite images. But the report did note that refugees, as well as fighters, have taken shelter in these ancient villages; some have even been living in the ancient tombs.

Most of the images from the ancient villages depict military compounds and tents erected within their walls. But the fighting has also brought damage to the Dead Cities. Here's one example:

A yellow arrow in the image above points to a standing ruin that was knocked down to make way for a new road.

Crac des Chevaliers and Qal’at Salah El-Din

These two structures are known as the Crusader Castles, and their still-standing ruins were the world's best examples of Crusader-era defense architecture. Crac des Chevaliers was rebuilt by the Hospitaller Order of Saint John of Jerusalem during the Crusades and then once again by the Mamluks in the 13th Century. Qal’at Salah El-Din, or the Fortress of Saladin, dates back to the 10th Century and stood in partial ruins prior to the recent conflict. They were added to the UNESCO list in 2006.

While AAAS didn't point to any reported damage to the fortress, the Crac des Chevaliers has been at the center of heavy fighting, and there were reports of gunmen in the castle in 2012, according to AAAS. Free Syrian Army fighters used the site, and the Syrian military shelled the castle and its historic chapel. Airstrikes and shelling continued through March 2014, until Syrian government forces recaptured the castle.

By October 2013, numerous craters (as noted by the yellow arrows) appeared at Crac de Chevaliars, including one on the south turret's roof.

The AAAS report describes moderate structural damage to the castle, including to its southeast tower and three visible craters to the northern part of the castle.