Insight - Tunnelling through triangle of death, Islamic State aims at Baghdad from south

Shi'ite volunteers secure the area from predominantly Sunni militants from the Islamic State, previously called the Islamic State in Iraq and the Levant (ISIL), in the desert region between Kerbala and Najaf, south of Baghdad, July 3, 2014. REUTERS/Alaa Al-Marjani

BAGHDAD (Reuters) - 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.