Smoke filled the air over the city centre, and emergency vehicles rushed to the scene.

Wednesday was the sixth anniversary of a powerful bomb attack at the UN headquarters in Baghdad's Green Zone that killed some 22 people. That bombing was the first large attack after US-led forces invaded Iraq and toppled the government of Saddam Hussein.

In recent months, attacks near the Green Zone have become rarer, and security has improved across the country. According to al-Jazeera's count, 275 people died in insurgent attacks in July, down from 437 the month before.