Violence appears to have increased sharply since U.S. troops left Iraq a month ago, as insurgents have unleashed a wave of furious bombings targeting Baghdad neighborhoods, Shiite pilgrims and police facilities in Sunni areas.

The deadly attacks have roots not only in the troops’ departure but also in a domestic political crisis that erupted in its wake. Shiite and Sunni leaders have squared off in a power struggle, one that analysts say insurgents are trying to turn into a full-scale civil war. How the politicians handle their own mess, and the attacks, will determine Iraq’s ability to hold itself together.

“They are not arbitrary attacks. They are sending messages that security is not under control,” said Wathiq al-Hashimi, an analyst in Baghdad who leads the Iraqi Group for Strategic Studies.

Hashimi and others said insurgents are also trying to exploit the political crisis between Shiite and Sunni leaders that has deadlocked the government. With each attack, the insurgents are trying to persuade militias from each sect to take up arms and start attacking each other.

Before their departure, U.S. troops had ceded control of the country’s security to Iraqi forces. But the Americans’ presence remained critical, analysts said, because they shared intelligence capabilities with their Iraqi counterparts and their visibility signaled that the Americans had influence over Iraq’s leaders.

After the U.S. exit, “what changed is a mind-set,” said Michael Knights, an Iraq expert at the Washington Institute for Near East Policy who has been a consultant to the Defense Department.“The U.S. was now definitely gone. Therefore, all bets are off.”

Knights worked in Iraq during the war and said commanders always assumed that Sunni extremists would launch at least some attacks after U.S. forces left. What amped everything up, he said, was the eruption of a sectarian political crisis that generated both fear and opportunity among insurgents. “It’s like a perfect storm,” Knights said.

In Washington, a senior Obama administration official said that although there had been several large attacks recently, they had not altered a pattern of cyclical violence over the past two years. Similar upticks, he said, occurred when U.S. forces exited Iraqi cities in 2009 and during other changes in the U.S. profile there.

“It’s always a mistake to look at these things in the isolation of one or two weeks,” said the official, who agreed to speak about internal Iraqi affairs on the condition of anonymity.

Referring to al-Qaeda in Iraq and other groups, the official said, “I imagine some of them saying to themselves, ‘The American troops are gone,’ and trying to create this narrative” that a security void has been created. “But it’s something they have tried on a regular basis at different points.”

He said it was “incumbent on Iraqi leaders to reel back in” the political turmoil so that insurgents “don’t have anything to use” in promoting sectarian strife.

Over 2,600 deaths in 2011

Indeed, in the past, spikes in violence in Iraq have often been followed by calmer periods.

Exact death tolls can be difficult to obtain in Iraq, in part because there are so many incidents throughout the country and because the number of attacks often increases during Shiite pilgrimages. An official at the Interior Ministry, which runs Iraq’s internal police forces, said 327 Iraqis have been killed in explosions or assassinations since Dec. 18, the day the last U.S. combat forces departed. The official spoke on the condition of anonymity because he was not authorized to speak.

For the year ending Dec. 31, Iraqi officials recorded about 2,640 deaths, according to totals from the Interior and Health ministries. A nonprofit group, Iraq Body Count, which has monitored civilian deaths since 2003, estimated that 460 civilians died violently after the troops’ departure, a 35 percent increase over monthly averages for last year. The 460 deaths is higher than the group’s monthly totals dating to August 2010, according to Iraq Body Count’s figures.

The totals are still far below those in 2006 and 2007, during the height of sectarian violence, when more than 2,000 Iraqis were killed in some months. And Iraqi officials say they are making progress against insurgents as they assume full control over their country.

“We have faith that our security forces have put the terrorist groups on the path to defeat,” said Ali Hadi al-Moussawi, a spokesman for Prime Minister Nouri al-Maliki.

‘Iraq will be tested’

U.S. officials never promised the transition would be easy. At a ceremony in Baghdad two days before the last troops left, Defense Secretary Leon E. Panetta sounded a note of caution.

“Let me be clear, Iraq will be tested in the days ahead — by terrorism, by those who would seek to divide, by economic and social issues, by the demands of democracy itself,” he said.

Even as Panetta was speaking, security forces loyal to Maliki, a Shiite, were moving to arrest Vice President Tariq al-Hashimi, a Sunni, on charges that he used his bodyguards to run a terrorism squad. Hashimi fled to a semiautonomous region of Kurdistan, where leaders provided refuge even as Maliki demanded his return to Baghdad to face trial. At the same time, Sunni leaders walked out of parliament and boycotted cabinet positions to protest what they said was Maliki’s move to create a Shiite-controlled dictatorship.

The morning of Dec. 22, starting at 6:30 a.m., insurgents ignited 15 bombs in Baghdad over two hours. Among their targets: Shiite neighborhoods, Sunni neighborhoods and a building that housed a government agency charged with rooting out corruption. More than 65 Iraqis were killed.

An al-Qaeda-affiliated group, the Islamic State of Iraq, claimed responsibility for the wave of bombings and a blast that followed four days later, according to the SITE Intelligence Group, a monitoring service. It said the attacks were carried out as revenge for the detention of Sunnis in Shiite prisons.

Sunni extremists are effectively taking on two groups, analysts say: the Shiite-controlled government and the Sunni elite. They also hope to convince the Sunni population that insurgent strikes are its only hope for obtaining power.

“I think Sunni militants are using the vacuum left by departure of U.S. troops to flex their muscles,” said Vali Nasr, a Tufts University scholar and Iraq expert. “The message is this: ‘While the Sunni political parties are bullied by Maliki, the Sunni insurgents are asserting Sunni claims to power.’ ”

Starting Jan. 5, insurgents attacked large groups of Shiites, killing more than 100, including pilgrims marching to holy shrines in southern Iraq. In recent days, militants have also attacked a police station and a security checkpoint in Anbar province, a predominantly Sunni area.

Meanwhile, the political stalemate continues in the background. Leaders talk about a national summit but argue about its agenda and when to hold it.

“We have a situation which the negotiating table only seems to invent more points of disagreement,” said Ramzy Mardini, a research analyst at the Institute for the Study of War.

Staff writer Karen DeYoung in Washington and special correspondents Asaad Majeed and Aziz Alwan in Baghdad contributed to this report.