A series of bombings targeting Shiite pilgrims, including by a suicide attacker disguised as a woman, killed 16 people in Baghdad Thursday despite heavy security across the capital.

The blasts are the latest in a protracted surge of nationwide bloodshed that has left more than 3,600 people dead this year, fuelling fears Iraq is slipping back into the brutal communal bloodshed that blighted the country in 2006 and 2007.

Three blasts -- two suicide bombings and a vehicle rigged with explosives -- targeted pilgrims who were preparing for commemorations for a revered figure in Shiite Islam.

In the deadliest attack, a militant dressed in a black full-length woman's robe, or abaya, blew himself up amid a group of Shiite worshippers in the west Baghdad neighbourhood of Mansur.

The explosion killed at least eight people and wounded 26, according to a police colonel and a medical source.

Another suicide attacker detonated a car bomb in Baab al-Sharji, central Baghdad, killing three more, while a vehicle rigged with explosives in the northern neighbourhood of Urr left five dead.

The worshippers -- many from elsewhere in the country -- were all walking from across the city to the district of Kadhimiyah, site of a shrine dedicated to Imam Musa Kadhim, the seventh of 12 revered imams in Shiite Islam.

The two-day rituals marking the anniversary of his death in 799 AD are due to climax on Saturday and Sunday.

Shiite pilgrims are often targeted by Sunni militants who regard them as apostates. In past years, multiple attacks have been carried out during the Imam Kadhim commemorations.

Due to the heightened threat of attack, the authorities have imposed heavy security measures on the capital, involving the closure of entire roads and barring certain vehicles from the streets.

Violence has surged in the past year to its highest level since 2008, while anti-government fighters control an entire city a short drive from Baghdad and parts of another.

The authorities blame external factors such as the civil war in neighbouring Syria for the unrest, and say that wide-ranging operations targeting militants are having an impact.

But the daily bloodshed has continued unabated, and diplomats and analysts say the Shiite-led government must do more to reach out to disgruntled minority Sunnis.

The latest attacks come as Iraq's political parties jostle to build alliances and form a government following elections last month that left incumbent Prime Minister Nuri al-Maliki in the driver's seat to remain in office for a third term.