Using secret tunnels built by Saddam Hussein and rough terrain to outfox Iraqi troops, Islamic State insurgents are getting dangerously close to Baghdad with the support of heavily-armed Sunni tribesmen, Iraqi security and intelligence officials said.
The al Qaeda offshoot, which poses the biggest security threat to Iraq since the fall of Saddam in 2003, has made new bold advances in the north, reaching a major dam and seizing a fifth oilfield and three more towns after routing security forces from the Kurdish autonomous region.
But some Iraqi intelligence and security officials are far more alarmed by the Islamic State's less heralded campaign in rural areas just south of the capital, rugged Euphrates valley terrain once known to U.S. forces as the "triangle of death".
While the Islamic State's march on Baghdad from the north has been halted near the town of Samarra 100 km (60 miles) from the city limits, the fighters have more quietly building up their forces on the capital's southern outskirts.
"We told the government that urgent military operations are essential to prevent the Islamic State from taking over further towns south of Baghdad; otherwise they will be very close to the capital," said Falah al-Radhi, head of a security panel in the provincial council of Hilla, the province just south of Baghdad.
For several weeks, the Sunni insurgents have been moving fighters, weapons and supplies from strongholds in western Iraq through secret desert tunnels to the town of Jurf al-Sakhar, about 60 km (40 miles) south of Baghdad.
Built by Saddam in the 1990s to hide weapons from U.N. weapons inspectors, the tunnels are also ideal hiding places that allow fighters to avoid military helicopters.
Islamic State militants occupying the city of Falluja and parts of Ramadi, where U.S. troops once faced a stubborn al Qaeda insurgency, access the tunnels from an area near military facilities once used by Saddam's troops.
"It makes it impossible for us to control this area," said an intelligence official, describing Jurf al-Sakhar and nearby towns just south of the capital.
The towns south of Baghdad and the surrounding lush, irrigated fields - a religiously mixed area where the mainly Sunni upper Euphrates valley meets the river's mainly Shi'ite lower reaches - formed one of the most violent parts of Iraq under U.S. occupation.
The territory, with its canals, ditches and thick vegetation, provides ideal cover for insurgents.
U.S. military and Iraqi security officials estimate the Islamic State has at least