Despite the fall of the self-proclaimed ISIS "caliphate" in 2019, the group continues to launch deadly attacks from hideouts in the Syrian desert, which extends from the outskirts of the capital Damascus to the Iraqi border.
The Observatory, which relies on a network of sources across the country, said the death toll could rise as most of the wounded soldiers were in a serious condition.
ISIS did not immediately claim responsibility for the attack.
Syrian soldiers are occasionally moved across country by bus, a high-risk mode of transport given the large concentration of men within a single unarmoured vehicle.
In October, 27 Syrian soldiers were killed when a bus in Damascus was blown up by two bombs. Two similar attacks in central Syria and in Damascus in January killed eight soldiers.
Palmyra, which has a Unesco World Heritage site that includes Roman ruins, was seized by ISIS in 2015 at the height of the country's civil war.
Palmyra, which means City of Palms, is known in Syria as Tadmor, or City of Dates. File photo from March 14, 2014, shows a sculpture in the ancient Syrian oasis city of Palmyra, 215 kilometres north-east of Damascus, displayed at the city’s museum. Joseph Eid / AFP Photo
Its name first appeared on a tablet in the 19th century BCE as a stopping point for caravans travelling on the Silk Road and between the Gulf and the Mediterranean. File photo from March 14, 2014 showing a partial view of the ancient oasis city. ISIL seized full control of Palmyra on May 21, 2015, putting the world heritage site at risk of destruction. Joseph Eid / AFP Photo
Palmyra rose to prominence during the Roman Empire. Photo from May 19, 2015 shows a road sign in Palmyra city. Reuters
Though surrounded by desert dunes, Palmyra developed into a luxurious metropolis thanks to the trade of spices, perfumes, silk and ivory from the east, and statues and glasswork from Phoenicia. Photo from May 19, 2015 shows residents walking in the city. Reuters
In the year 129AD, Roman emperor Hadrian declared Palmyra a ‘free city’ within his empire. A general view taken on May 18, 2015. AFP Photo
As the Roman Empire faced internal political instability in the third century, Palmyra took the opportunity to declare its independence. File photo from March 14, 2014 shows the citadel of the ancient Palmyra. Joseph Eid / AFP Photo
Palmyra bears scars of Syria’s continuing war: clashes between armed rebels and government forces in 2013 left collapsed columns and statues in their wake. Days before its capture, a Syrian armoured tank took up position during battles against ISIL on May 19, 2015. EPA
According to the governor of Homs province, the inner city is home to about 35,000 people, including those displaced by fighting nearby. Photo handout released on May 17, 2015, by the Syrian official news agency Sana, shows a wide view of the ancient city. Sana via AP
Another 35,000 live in the city’s suburbs. Seen here on May 19, days before its fall, residents walk near a military lorry that belongs to forces loyal to Syria’s president Bashar Al Assad, near the historical city of Palmyra. Reuters
Hundreds of statues and artefacts from Palmyra’s museum were transferred out of the city before it fell to ISIL, according to Syria’s antiquities chief Mamoun Abdulkarim. Photo from May 19, 2015 shows Palmyra’s famous graves. Reuters
Many other priceless archaeological sites, including massive tombs, could not be moved, renewing fears the extremist group will destroy the ruins if they reach them. Photo released on May 17, 2015, by the Syrian official news agency Sana. Sana via AP
The city was retaken in March 2016 after Russia's intervention to assist President Bashar Al Assad, only to fall back into ISIS hands in December 2016, before again being recaptured by government forces in January 2017.