A series of car bombs near Shiite mosques targeting worshippers attending weekly prayers killed at least 18 people on Friday, the latest in a spike in unrest ahead of Iraq's first polls since 2010.

The blasts, which also wounded more than 100 people, struck within an hour of each other in the Baghdad neighbourhoods of Binook, Qahira, Zafraniyah and Jihad, as well as in an area of southern Kirkuk city.

No group immediately claimed responsibility for the attacks, but Sunni militants linked to Al-Qaeda frequently target Shiite Muslims whom they regard as apostates and supporters of the Shiite-led government.

Overall, violence in Iraq on Friday left 21 people dead and 109 wounded.

In Baghdad, four car bombs were detonated near Shiite mosques across the capital, leaving at least 14 people dead and 35 wounded, security and medical officials said, speaking on condition of anonymity.

And in Kirkuk, 240 kilometres (150 miles) north of the capital, four people died and 72 were hurt by another car bomb targeting a Shiite mosque, provincial health chief Sadiq Omar Rasul said.

"I was praying inside the Husseiniyah (Shiite mosque) during Friday prayers," said Salim Aziz al-Bayati, a worshipper in Kirkuk who was wounded.

"Then, all of a sudden, a great, horrible explosion happened, and the roof fell on our heads."

Also wounded was Mohsen al-Battat, a representative of Iraq's most revered Shiite cleric Grand Ayatollah Ali Husseini al-Sistani, who had been leading the prayers.

Bayati continued: "I saw the imam (Battat) was lying on the floor, and blood was on everyone's bodies. When they (paramedics) were taking me away, I saw a fire covering the Husseiniyah, and nearby houses and cars."

Later on Friday in Kirkuk, three separate shootings left a construction worker dead and two soldiers wounded, officials said.

In Baghdad's Qahira neighbourhood, an AFP reporter described seeing pools of blood and massive damage to nearby cars, houses and shops. Several residents were crying as soldiers imposed a heavy security presence in the area.

Security forces elsewhere in the capital threatened to detain AFP journalists for attempting to film and take photographs of the aftermath of the bombings.

Also on Friday, a gunman killed two primary school teachers who were gathering materials for a demonstration against the local education department in the town of Kut, south of Baghdad.

The attacks come amid a spike in violence nationwide as the country prepares for its first elections in three years -- provincial polls that will be held in 12 of Iraq's 18 provinces on April 20.

Questions have been raised over the credibility of those polls as elections have been postponed in two provinces roiled by months of protests, and at least 11 candidates have been killed.

The vote is seen as a key barometer of support for Prime Minister Nuri al-Maliki as he grapples with criticism from within his unity cabinet and protests in the minority Sunni Arab community.

The attack in Kirkuk is also likely to raise tensions in a city at the heart of a long-running dispute over territory between the central government in Baghdad and the country's autonomous Kurdistan region.

The unresolved row is often cited by officials and diplomats as the biggest long-term threat to Iraq's stability and, at its worst, Iraq's parliament speaker warned it could lead to civil war.

Iraq's military and police are consistently described by Iraqi and American officials as capable of maintaining internal security, but are not yet fully able to protect the country's borders, airspace and maritime territory.

And while violence is down from its peak in 2006 and 2007, attacks remain common, with this month's death toll of 262 already the highest since August, according to figures compiled by AFP.

The latest unrest comes days after the 10th anniversary of the US-led invasion which ousted dictator Saddam Hussein and had sought to establish a stable, democratic ally in his place but instead unleashed brutal violence and endless political disputes.