The CIA and US special operations forces have launched a secret campaign to hunt terrorism suspects in Syria as part of a targeted killing programme that is run separately from the broader US military offensive against Isis (Islamic State).
The CIA and the Joint Special Operations Command are both flying armed drones over Syria in a collaboration responsible for several recent strikes against senior Isis operatives, according to US officials.
Among those killed was a British militant believed to be an architect of the group's effort to use social media to incite attacks in the US.
The clandestine programme represents a significant escalation of the CIA's involvement in the war in Syria, enlisting the agency's powerful Counterterrorism Centre.
But while the centre has been given an expanded role in identifying and locating senior Isis figures, US officials said the strikes were carried out exclusively by the special operations command. They said the programme was aimed at terrorism suspects deemed "high value targets".
"These people are being identified and targeted through a separate effort," said a senior US official, referring to the British militant, Junaid Hussain, and others killed in recent weeks.
The decision to enlist the CIA and special operations command reflects rising anxiety among US counterterrorism officials about the danger Isis poses, as well as frustration with the failure so far of conventional strikes to degrade its strength.
Syria is a new front in a spreading campaign of secret operations and drone strikes that already encompasses Pakistan, Yemen, Somalia and parts of North Africa.
Officials said the co-operation between the CIA and special operations command in Syria was increasingly seen as a model that could be employed in future conflicts.
The programme has accounted for only a handful of strikes so far, a tiny fraction of the more than 2450 conducted in Iraq and Syria in the past year. That broader US-led assault has relied on conventional bombs to dislodge Isis from territory it has seized.
The joint programme is aimed primarily at leadership figures in Isis as well as operatives suspected of being involved in efforts to build a terror network beyond the borders of its declared caliphate. Al-Qaeda militants are also approved targets.
Hussain, the 21-year-old British militant killed last month, was moved towards the top of the target list after being linked to one of two gunmen killed in Garland, Texas, this year after opening fire at a cartoon contest that invited participants to draw pictures of the Prophet Mohammed.
Hussain is not known to have been involved in Isis' gruesome beheadings. The decision to kill him makes clear that even militants only involved in Isis' media efforts are regarded as legitimate US targets.
World heritage sites in danger
The ancient city was once one of the most well-known tourism spots in all of Syria. The site, which predates Islam by hundreds of years, had become a centre for trade by the 1st century AD - its existence is even recorded in biblical texts. It has been controlled by Isis since May.
The Iraqi fortress city of Hatra is believed to date back to the Parthian empire in the 3rd or 2nd century BC, and it later became the capital of the first Arab Kingdom. Video released in March by Isis showed the group using sledgehammers and even guns to destroy carvings and statues.
3 Ashur (Qal'at Sherqat)
Ashur, also known as Assur, is a city in modern-day Iraq that dates back to the third millennium, and it later became the first capital of the Assyrian Empire. The city was associated with the god Ashur and became an important religious city. In May, there were a number of reports that the city's ancient arches had been blown up by militants.
Located at an important point along trading routes since the 2nd millennium BC, Aleppo has had a rich history and has the architectural legacy of a variety of different empires, religions and time periods. The city has various buildings of historical importance, including its famous citadel, a large fortified palace that dates back thousands of years. Since the Syrian war began in 2012 it has been divided between rebel forces and government troops. A number of important sites in the city have been damaged including the famous minaret at the 11th century AD Great Mosque of Aleppo, which was destroyed in 2013.
5 The ancient villages of Northern Syria
These villages, also known as the "Dead Cities", are in the northwest and date back to between the 1st-7th centuries, but they were all abandoned by the 10th century. Their ruins present a picture of life in the Antiquity and the Byzantine periods.
The archaeological city of Samarra in Iraq, once the capital of the Abbasid Caliphate, is considered an especially important historical site by Unesco as it is the "only surviving Islamic capital that retains its original plan, architecture and arts, such as mosaics and carvings". The city is well-preserved as it was abandoned relatively early, and only 20 per cent of it has been excavated so far.
7 The Crusader castles
Crac des Chevaliers and Qal'at Salah El-Din are two castles in Syria that date to the period of the Crusades and represent an important mixture of European and Near Eastern influences. However, both have been the scene of heavy fighting: In 2013, rebels said they had just managed to defeat regime troops fighting in the walls of the Crac des Chevaliers, though the Syrian army retook the castle in 2014.
8 Ancient city of Damascus
Damascus is the capital of Syria and one of the oldest continually inhabited cities in the world, with some excavations showing the city was inhabited as early as 8000 to 10,000 BC. The city became the capital of the Umayyad Caliphate and has been important to Arab culture since. Within the walls of the Old City, Unesco says there are 125 protected monuments, including the Umayyad mosque, still one of the largest mosques in the world.
9 Ancient city of Bosra
Bosra was once the capital of the Roman province of Arabia and still features a 2nd-century Roman theatre within the Old City walls. It contains a number of monuments from the Nabataean, Byzantine and Umayyad periods, too. The city has been the site of considerable fighting during the Syrian civil war.