A car bomb has exploded in a largely Shia neighbourhood of central Baghdad, killing at least 15 people, police say.
Kirkuk was one of four Iraqi city targeted by car bombers
The blast came as US Defence Secretary Robert Gates was on a visit to the Iraqi capital. He was meeting Iraqi leaders in the nearby Green Zone.
Bursts of gunfire were heard after the blast and a thick plume of black smoke rose from the Karrada area.
Earlier in the day three parked car bombs exploded in other Iraqi cities, killing at least eight people.
One of the bombs killed one person in Mosul where Mr Gates landed.
Bombings killed five people in Baquba, north-east of Baghdad, and at least two in the northern city of Kirkuk.
The Baghdad bomb went off near a mosque as people gathered for evening prayers, Reuters news agency reports.
The blast also came just before Mr Gates held a news conference in the heavily fortified Green Zone across the River Tigris.
Despite the violence, Mr Gates said he believed that "a secure, stable Iraq is within reach".
It is the sixth time Mr Gates has visited Iraq in a year - in September he accompanied President George W Bush.
Pentagon press secretary Geoff Morrell said Mr Gates wanted to "get [the military's] take on the situation and to see what more they can do to capitalise on the gains that have been made since the surge of US forces in Iraq".
Mr Gates met Iraqi leaders including Prime Minister Nouri Maliki, as well as the US military commander in Iraq, General David Petraeus.
The US is starting slowly to reduce the number of combat brigades in Iraq after this year's surge in troop levels, which appears to have succeeded in helping stem violence in the centre of the country.
Iraqi officials say the monthly death toll among civilians and police is falling, with 606 killed in November compared with 887 in October.