Story highlights Four of the five blasts occurred in the Shiite heartland in the southern region of Iraq

Another was in Mahmoudiya, a predominantly Sunni area just south of Baghdad

No one immediately claims responsibility

Iraq has seen an uptick in violence in recent months

The longstanding bad blood between Sunnis and Shiites in Iraq boiled Monday amid another round of attacks: the killing of 25 people and wounding of dozens more in five car bombings.

Four of the blasts occurred in the Shiite heartland in the southern region of Iraq.

Two car bombs exploded near a busy outdoor market in Amara, killing 13 people and wounding 24 others. A bomb went off near an outdoor market in Diwaniya, killing six people and wounding 20 others. And two people died and 11 others were wounded in a bombing at a commercial area in Karbala.

Another blast occurred in Mahmoudiya, a predominantly Sunni area just south of Baghdad. Four people were killed and 14 others were wounded in that attack.

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No one immediately claimed responsibility for the attacks, nor was it clear whether they were related.

But the fighting has prompted fears among Iraqi leaders and international powers that the tensions between Sunnis and Shiites could escalate and bring a return of the full-blown sectarian war that raged last decade.

Since the overthrow of Saddam Hussein's government last decade, Sunni Arabs have been politically marginalized and Shiites, who represent a majority of Iraqis, have emerged with more power.

There have been protests for months by Sunni Arabs against the Shiite-led government and its prime minister, Nuri al-Maliki. The anger has escalated since a confrontation last week between police and protesters in Hawija.

The International Crisis Group last week said that the "failure to integrate Sunni Arabs into a genuinely representative political system in Baghdad risks turning Iraq's domestic crisis into a broader regional struggle."

"The most urgent task today is to tamp down the flames, and the burden for this lies above all with the government," the Belgium-based think tank said in a report.

"The country is at a crossroads," said Martin Kobler, U.N. special representative in Iraq.