Iranian-backed militants recently received specialist training inside Iran to help mount an attack on American military bases in Iraq, the top US commander in Baghdad said on Tuesday.
General Ray Odierno detailed a plot in which he said Ketaib Hezbollah, a Tehran-sponsored Shiite insurgent group, had planned an attack in recent weeks, prompting the US army to increase its security measures.
Iran plays the battlefield in Iraq
"There has been some intelligence of Iranian surrogates attempting to attack US bases, which we are watching very carefully," Odierno told reporters at Camp Victory, a giant US base west of the capital.
"In the last couple of weeks there has been an increased threat of a potential Iran attack ... so we have increased our security in some of our bases," said the general, who commands all 74,000 U.S. troops stationed in Iraq.
"This is another attempt by Iran and others to attempt to influence the US role here inside of Iraq," he added, naming Ketaib Hezbollah as the militant group behind the conspiracy, and describing the threat as "consistent".
The US military maintains that Shiite militant groups backed by Iran have in recent years been used to bolster Tehran's influence in Iraq, which remains without a new government four months after an inconclusive general election.
The disclosure of a plan to attack American military facilities comes as Washington withdraws combat troops from the country.
Odierno declined to say which U.S. bases had been targeted and said it was unclear if the Tehran government was directly involved in the plot.
"It's convoluted. It's always very convoluted," said the general when asked if the Iranian leadership had played a role.
"What we do know is the people getting ready to conduct this attack went back, got special training in Iran, they came back and we know that there were experts sent from Iran into Iraq to help them to do this in the last month or so."
U.S. still eyes its Iraq exit in 2011
Odierno said the threat would not disrupt an ongoing drawdown of US combat forces that will see American combat troops leave Iraq by September 1, after which a 50,000 training and advisory force will remain until December 2011.
"In my mind it won't impact," he said. "Our assessment is that the Iraqi security forces are capable of maintaining a level of stability necessary for Iraq to continue to move forward politically and economically.
"Fifty thousand US soldiers on the ground is still a significant capability and we still can do many things with 50,000 soldiers," he added.
The number of Iraqis killed in violence -- 284 people, comprising 204 civilians, 50 police and 30 soldiers -- in June was sharply down year-on-year, according to figures released by the health, defence and interior ministries.
The number killed was one third less than the 437 people who died last June, when bombings in the lead-up to the withdrawal of US forces from Iraq's towns and cities resulted in the bloodiest death toll seen in 11 months.
Odierno said levels of violence were falling overall because the insurgents' "ability to surge is significantly less" because of Iraqi and US operations in recent months that had damaged their leadership structure.
"We have seen no communications between Al-Qaeda in Iraq and the leadership in Afghanistan and Pakistan," he added. "We think this is fairly significant."
However, he cautioned that Al-Qaeda still had operatives in the desert lands between central Iraq and the Syrian border and indicated that the levels of violence were still far too high.
"The bottom line is we still have Muslims killing Muslims. We still have Iraqis killing Iraqis... We haven't eliminated that environment."