At least 78 people were killed Thursday in a series of attacks on Shiites in Iraq, marking the deadliest day since U.S. troops withdrew last month and raising new worries about the country's sectarian divisions.

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BAGHDAD — At least 78 people were killed Thursday in a series of attacks on Shiites in Iraq, marking the deadliest day since U.S. troops withdrew last month and raising new worries about the country’s sectarian divisions.

The bombings Thursday in Baghdad and outside the southern city of Nasiriyah were the second major wave of attacks since the departure of the last U.S. troops from Iraq less than three weeks ago.

Sectarian tension has escalated as a political dispute threatens to unravel U.S.-backed power-sharing arrangements involving the country’s Shiites, Sunni Arabs and ethnic Kurds.

The attacks, which wounded at least 147 people, occurred against the backdrop of a deepening political standoff between the country’s Shiite and Sunni leaders.

A majority of the Sunni-supported Iraqiya political bloc boycotted parliamentary proceedings Thursday. Members of the group accuse Prime Minister Nouri al-Maliki, a Shiite, of trying to dissolve a political framework established under U.S. guidance to ensure power-sharing among Shiites, Sunnis and Kurds.

There was no immediate claim of responsibility. But Sunni insurgents linked to al-Qaida frequently targeted Shiites with coordinated bombings during the violence that pushed the country to the brink of civil war five years ago.

“Definitely … there is a relationship between these explosions and the political crisis, but it doesn’t mean necessarily that one of the sides in the crisis is directly responsible,” said Dhiya Shikerchi, an Iraqi political analyst.

Shikerchi raised the prospect that a separate entity was trying to exploit political tensions to return the country to sectarian strife.

There is no sign that Shiite militias are taking the bait. Instead, the political movement of Shiite cleric Muqtada al-Sadr, whose militia was blamed for near-daily kidnappings and killings at the height of the sectarian fighting, called on government forces to improve their security plans.

The attacks began during Baghdad’s morning rush hour when explosions struck the capital’s largest Shiite neighborhood of Sadr City and another district that contains a Shiite shrine, killing at least 30 people, according to police.

Several hours later, a suicide attack hit pilgrims heading to the Shiite city of Karbala, killing 48, police said. The explosions took place near Nasiriyah, about 200 miles southeast of Baghdad.

Hospital officials confirmed the deaths. Authorities spoke on condition of anonymity.

The blasts occurred in the run-up to Arbaeen, a holy day that marks the end of 40 days of mourning following the anniversary of the death of Imam Hussein, a revered Shiite figure. During this time, Shiite pilgrims — many on foot — make their way across Iraq to Karbala, south of Baghdad.

Baghdad military spokesman Maj. Gen. Qassim al-Moussawi said the aim of the attacks is “to create turmoil among the Iraqi people.”

Two weeks earlier, militants killed at least 69 people as a wave of bombs ripped through mostly Shiite neighborhoods in Baghdad. An al-Qaida front group in Iraq claimed responsibility.

The risks of sectarian conflict go well beyond Iraq. Turkey’s Anatolia news agency quoted that country’s foreign minister, Ahmet Davutoglu, as warning that Sunni-Shiite tensions could throw the Middle East into a “regional Cold War.” Davutoglu’s comments this week were published on the eve of a visit to Iraq’s powerful neighbor, predominantly Shiite Iran.

In Iraq, fear of retaliation has prompted some Sunnis to flee predominantly Shiite neighborhoods of Baghdad, a reminder of the worst days of the sectarian strife that eased only after the U.S. military increased its troop strength and formed alliances with Sunni tribal leaders to fight al-Qaida and its affiliates.

Despite concerns about Iraq’s stability, the Obama administration could not reach agreement al-Maliki’s government about a continuing U.S. military role in Iraq. Under an agreement reached by the Bush administration, the last U.S. troops left Iraq on Dec. 18.

National Security Adviser Faleh Fayadh sought to deflect criticism that Iraq’s security forces weren’t up to the task, telling the U.S.-funded Alhurra satellite-television network that there were bigger attacks when U.S. troops were present.

Material from The Associated Press and The Washington Post is included in this report.