The United States and Iraq are negotiating a possible new security deal that would keep US forces in the country beyond a December 31 deadline for withdrawal, the top US military officer said.
Admiral Mike Mullen's comments marked the first high-level confirmation from the US military that talks were underway on the politically-charged subject, which faces some stiff opposition in Iraq.
"The negotiations are ongoing and it's hard," Mullen told reporters.
He said the discussions were addressing both the size of a possible US military mission as well as the capabilities that Iraqi forces lacked.
"There are very clear capability gaps the Iraqis are going to have," said Mullen, citing air power, air defense and intelligence analysis.
"And both the Iraqi security forces and our forces recognize those gaps are there," he said at a Pentagon Press Association luncheon.
How those gaps would be addressed is "at the heart of the discussions and negotiations which are ongoing as we speak," the admiral added.
Mullen would not speculate on the optimum number of US troops that should stay on beyond this year, but added: "it's what the Iraqi government and really the Iraqi people say is acceptable to them to provide for their own security."
About 46,000 US troops remain in Iraq and the entire force is due to leave by December 31 under a security agreement with Baghdad.
But top US officials have said that they would consider keeping some troops there after the deadline if requested by Iraqi authorities.
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Iraqi commanders acknowledge their military remains heavily dependent on US logistical support, air power, equipment and expertise, while some Baghdad politicians are anxious to retain American troops as a peacekeeping force in reserve.
President Barack Obama's administration has offered to keep up to 10,000 forces in Iraq next year, The Washington Post reported this week, citing US officials.
Such a move would be politically fraught both in the United States and in Iraq, where some view US forces as unwelcome.
Anti-US Iraqi Shiite cleric Moqtada al-Sadr has threatened to launch attacks against American forces if they stay beyond the year-end deadline.
In April, Sadr threatened to reactivate the Mahdi Army, the militia he formally disbanded in 2008, if US forces do not withdraw by December 31.
The once-powerful Mahdi Army, which fought against Iraqi and US-led coalition forces between 2004 and 2007, has been identified by Washington as the main threat to stability in Iraq.
Before it was disbanded, the militia numbered some 60,000 fighters swearing fierce loyalty to Sadr.
In Washington, Obama also risks alienating some members of his own party if he backs off the withdrawal deadline, as many Democrats in Congress are anxious to wrap up the wars in Iraq and Afghanistan.
The discussions on a possible extended US military mission come amid concerns in Washington over Iran's role in Iraq, and Mullen said any new security agreement would need to take Tehran into account.
Mullen accused Iran of increasing its support for Shiite militants in Iraq after having curtailed such activities in 2008.
Iran was supplying more sophisticated and more lethal weapons to the groups, including makeshift rocket launching systems and armor-piercing shaped charges, that were being used against American troops.
"Iran is very directly supporting extremist troops which are killing our troops," Mullen said.