Several US senators have accused Iraqi Prime Minister Nuri al-Maliki of promoting "a sectarian and authoritarian agenda" just as the political leader pays a visit to Washington this week.

In a letter sent Tuesday to President Barack Obama, six senators expressed alarm at the "deteriorating situation in Iraq" and urged Obama to expand counterterrorism assistance while pressing the Shiite prime minister to reconcile with Sunni and Kurdish leaders.

"Unfortunately, Prime Minister Maliki's mismanagement of Iraqi politics is contributing to the recent surge of violence" in Iraq, where 7,000 civilians have been killed this year, wrote the four Republican and two Democratic lawmakers.

"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 was signed by Republican Senators John McCain, James Inhofe, Bob Corker and Lindsey Graham, along with Democrats Carl Levin and Robert Menendez.

Maliki's failures were pushing many Sunnis "into the arms of Al-Qaeda in Iraq and fueling the rise of violence, which in turn is radicalizing Shia Iraqi communities and leading many Shia militant groups to remobilize," the senators said.

They warned the situation threatened to pitch Iraq into an all-out civil war.

Maliki's visit presented a chance to remind Americans of the strategic importance of Iraq and to convey to the prime minister the US government's grave concerns about sectarian violence and Iran's influence, according to the letter.

"We urge you to make clear to Prime Minister Maliki that the extent of Iran's malign influence in the Iraqi government is a serious problem in our bilateral relationship, especially for the Congress," the senators wrote.

They pointed to reports that Iran is using Iraqi airspace to ferry weapons and aid to Syria's embattled regime.

Before departing Baghdad for the United States, Maliki's spokesman Ali Mussawi told AFP that the Iraqi president will seek to "intensify efforts to gain American support for fighting terrorism" during his visit.

Maliki was due to meet Obama at the White House on Friday, nearly two years since the withdrawal of US troops from Iraq.

The Americans pulled out after talks on a long-term military presence collapsed over the issue of legal immunity for US forces, which Iraq opposed.

Despite the sharp criticism of Maliki, the senators urged the Obama administration to "step up our counterterrorism support for Iraq," including through more intelligence sharing.

But they said that Maliki needed to show that any US security assistance was part of a broader strategy to address the political grievances feeding the current violence.

Obama should convey to Maliki that if he devises a "real governance strategy" in Iraq, the United States would be prepared to provide "appropriate support," the senators said.

Iraq has ordered billions of dollars of US military hardware and an order for dozens of F-16 fighter jets has been a source of dispute between the two countries, with Baghdad calling for faster delivery.