Saddled with Middle East problems ranging from Iran to Syria and beyond, President Barack Obama now faces one that is both old and new: Iraq.

Mounting sectarian tensions, inflamed by the raging civil war in neighbouring Syria, have combined to send violence in Iraq to its highest level since Obama withdrew the last US troops in December 2011, US officials and Middle East analysts say.

A Sunni Muslim insurgency against the Shia-led Baghdad government has also been reawakened.

The deteriorating situation — largely overshadowed by a Syrian civil war that has killed over 80,000 people — has prompted what US officials describe as an intense, mostly behind-the-scenes effort to curb the violence and get Iraqis back to political negotiations.

The United States spent hundreds of billions of dollars and lost nearly 4,500 soldiers during an eight-year war in strategically placed, energy-rich Iraq.

In the wake of the removal of the Saddam Hussein regime, Iraqis have failed to agree on a permanent power-sharing agreement, threatening the country's long-term stability.

US Vice President Joe Biden, who has been Obama's point man on Iraq, called Iraqi Prime Minister Nouri Al-Maliki, Iraqi Kurdish leader Massoud Barzani and Osama Nujayfi, the head of Iraq's parliament, in a round of calls Thursday and Friday, the White House said.

To Maliki, the vice president "expressed concern about the security situation" and "spoke about the importance of outreach to leaders across the political spectrum," Biden's office said in a statement Friday.

US diplomacy is aimed in part at persuading Maliki, a Shia, and his security forces not to overreact to provocations. Maliki's opponents accuse him of advancing a sectarian agenda aimed at marginalising Iraq's other sects and cementing Shia rule.

The latest leap in violence began in late April at a Sunni protest camp in Hawija near the sensitive city of Kirkuk, where a clash between protesters and Iraqi security forces killed more than 40 people.

A US official said the Obama administration was "very actively engaged" after the Hawija clash in preventing a further escalation. Washington urged Iraqi forces not to use firepower, but to settle outstanding issues with local tribal leaders.

"I don't want to exaggerate our influence, but this is the kind of stuff we do behind the scenes," said an official who spoke on condition of anonymity. "When there is a real crisis, they all run to us ... We're a neutral party."

Others say Washington's influence in Iraq, which began waning even when US troops were still there, has plummeted.

"What is lacking is confidence of trust among the politicians," Iraqi Foreign Minister Hoshyar Zebari told CNN Tuesday. "And we have lost the service of an honest broker. Before, it used to be the United States."

'Zombie insurgency'

Most worrying to US officials and analysts who follow Iraq closely is the rebirth of the Sunni insurgency.

"What you're really looking at here is a kind of zombie insurgency — it's been brought back to life," said Michael Knights of the Washington Institute for Near East Policy, who has studied Iraq and travels there frequently.

By his count, violent incidents have escalated to about 1,100 a month from 300 monthly at the end of 2010.

After the Hawija clashes, the US official said, "For the first time really in a few years, we saw people with their faces covered and RPGs (rocket-propelled grenades) and heavy weapons coming into the streets in a very visible way."

The official called the increase in suicide bombings, allegedly by Al-Qaeda in Iraq, "very concerning," adding that such insurgent groups could "wreak havoc" on political efforts to solve the conflict.

"I wouldn't call it a strategically significant increase, yet," the official said of the violence. "We're in this post-civil war, pre-reconciliation interregnum, gap, period, in which Iraq can tilt either way."

The setbacks in Iraq have revived criticism from those who opposed Obama's decision to withdraw all US troops from the country, rather than leave behind a residual force. The White House has said it could not secure political agreement from Iraq's Sunni, Shia and Kurds for a law allowing a continued troop presence.

At a Senate hearing last month, Senator John McCain, who opposed the troop withdrawal, asked Assistant Secretary of Defense Derek Chollet how things turned out in Iraq. McCain, an Arizona Republican, cited Obama's dictum that "the tide of war is receding."

"I think Iraq is more stable today than many thought several years ago," Chollet replied.

"Really? You really think that?" McCain pressed. When Chollet said he did, the senator shot back, "Then you're uninformed."

The violence, which includes confrontations around the Sunni protest movement, near-daily car bombings and attacks on mosques, is nowhere near the levels of 2006-2008.

Still, Kenneth Pollack, a former CIA and White House official now at the Brookings Institution's Saban Centre for Middle East Policy, said: "I think we're going to see great sectarian violence. The question is, how bad does it get?"

Syria as 'accelerant'

Syria's increasingly sectarian civil war, pitting mostly Sunni rebels against the Alawite (Shia) government of President Bashar Al-Assad, is not the prime cause of Iraq's troubles, officials and analysts said.

Iraq's failure to find a stable power-sharing deal among the country's ethnic and sectarian groups is to blame, they said. Iraq's Sunnis feel excluded and threatened, and have been staging mass protests since December.

But Syria's war "is an accelerant" in Iraq, Pollack said.

"We're seeing both Shia and Sunnis going over to fight" in Syria, the US official said. "It's kind of encouraging this sectarian polarisation in a way."

Iraqis follow the Syrian conflict closely via YouTube video clips, he said.

Sunnis see the violence perpetrated by Al-Assad's government, he said, whereas Iraq's Shia see gruesome excesses perpetrated by the rebels.