A series of attacks across Iraq have killed at least 20 people, including 15 who perished in bombings at markets in the capital, officials have said.

At least 12 people were killed on Friday when a car bomb went off in Baghdad's mainly Shia Zafaraniyah neighbourhood, leaving 34 others injured and several shops damaged, sources told Al Jazeera.

Another bomb went off near an outdoor market in the Shia district of Obeidi, killing three shoppers and wounding 12, police said.

Earlier, a roadside bomb targeted a convoy of Shia militiamen, killing four fighters and wounding seven in the area of Iskandariyah, south of Baghdad, the Associated Press news agency reported.

Shia militiamen have joined Iraq's armed forces in their battle against the Islamic State group, which has seized large swaths of land in the country's north and west.

In northern Iraq, residents said Islamic State fighters killed Maiser al-Waqaa, a Sunni tribal chief, along with two brothers in the village of al-Houd, just south of Mosul.

They said al-Waqaa ran in parliamentary elections earlier this year but failed to win a seat.

Fight against IS

Sunni rebels captured Mosul, Iraq's second largest city, and overran much of northern Iraq in June.

The Islamic State group has declared a "caliphate" straddling the Iraqi-Syrian border.

Meanwhile, the United States on Friday called for the creation of a broad international coalition against the Islamic State in Iraq and Syria.

"There is no time to waste in building a broad international coalition to degrade and, ultimately, to destroy the threat posed by ISIL," Secretary of State John Kerry and Defence Secretary Chuck Hagel said in a joint statement.

The US has conducted air strikes in northern Iraq in recent weeks, allowing Kurdish and Iraqi forces to regain ground lost against the Islamic State fighters.