The United States vowed Wednesday to help Iraq combat terror groups as mounting attacks claimed more lives ahead of talks between Iraqi Prime Minister Nuri al-Maliki and President Barack Obama.

Maliki's visit to Washington comes as his country is wracked by the worst unrest since 2008, and just a few weeks before the two-year anniversary of the withdrawal of American troops from Iraq.

Only hours before he met with Vice President Joe Biden for talks focusing heavily on security, three suicide bombings killed 14 Iraqi security forces, the deadliest in a series of attacks which have left 35 dead in two days.

"Vice President Biden reiterated the US commitment to equip Iraqis to fight Al-Qaeda, and Prime Minister Maliki made clear that he views the United States as Iraq's security partner of choice," the White House said in a statement, calling the talks "friendly" and "constructive."

Unrest in Iraq has reached a level unseen since 2008 and killed more than 5,400 people this year, with Iraqi authorities so far failing to curb daily attacks despite a swathe of operations and tightened security measures.

Although Iraq has already received some $14 billion in US support, Maliki has said he intends to press Washington for more help when he meets with Obama on Friday at the White House.

"We do want to help the Iraqis develop the capability to target these networks effectively and precisely," a senior administration official told reporters after the two-hour breakfast meeting.

"It is a fact now that Al-Qaeda has a presence in western Iraq, and it has a presence in terms of camps and facilities and staging areas that the Iraqi forces are unable to target effectively," the US official said, asking not to be named.

Many militants are slipping into Iraq from Syria, armed with heavy weapons, and targeting Iraqi forces as well as civilian Shiite areas. "They're targeting playgrounds, weddings, funerals, and this is having a devastating psychological impact," the official said.

Washington had "a pretty good handle now on the Islamic State of Iraq and the Levant networks and where they are and where it's coming from," the US official said.

In a bid to clamp down on the attacks, the US was "increasing expert cooperation with the Iraqis... to help the Iraqis have a better vision of what they face so they can target it effectively."

US officials have also already notified Congress of plans to sell Iraq "a major air defense system which allows them for the first time to take sovereign control of their air space, which right now they don't have," the official said.

He refused to go into details, but the Washington Post reported that Baghdad was hoping to buy US-made Apache helicopters.

Iraq has also ordered dozens of F-16 warplanes from the United States which were on track for delivery in late 2014, the US official said, confirming Baghdad had now deposited some $650 million as a down payment for the planes.

He insisted though that the solution to combating groups such as the Islamic State of Iraq and the Levant did not just lie in weapons sales.

"What we don't want the Iraqis to do is to take just a security-centric approach to this. This is an asymmetrical threat and it has to be approached asymmetrically," the US official said.

"What that means is making sure they have information in terms of where people are located, where it's coming from, where the funding is coming from, and that's something that we can do pretty effectively."

Pointing to past efforts to win over Iraqi tribes to the side of the government, he said that in their talks the Iraqis were "keen to develop the same type of approach."

Maliki was heading for talks with lawmakers at the US Congress later Wednesday, a day after a group of US senators accused him in a letter to Obama of contributing to an alarming slide back into a sectarian war.

"By too often pursuing a sectarian and authoritarian agenda, Prime Minister Maliki and his allies are disenfranchising Sunni Iraqis, marginalizing Kurdish Iraqis, and alienating the many Shia Iraqis who have a democratic, inclusive and pluralistic vision for their country," the letter said.

Maliki's failure was pushing Sunnis "into the arms of Al-Qaeda in Iraq," the six Republican and Democratic senators said.