The White House and the US military scrambled to play down a suggestion by the nation's top officer that deploying ground forces in Iraq to fight Islamic State jihadists was an option.
General Martin Dempsey, chairman of the US Joint Chiefs of Staff and President Barack Obama's top military advisor, had said US military advisors could be sent into combat alongside Iraqi forces.
Dempsey said the US personnel could "provide close-combat advising," but the White House insisted the idea of US troops in battle was a "purely hypothetical scenario."
And in a day of mixed messages, Colonel Ed Thomas, a spokesman for the general, then tried to bring some clarity to the situation, saying in a rare statement that Dempsey "doesn't believe there is a military requirement for our advisors to accompany Iraqi forces into combat."
Military leaders nevertheless warned of a further escalation in their battle against the jihadists, just as two branches of the rival Al-Qaeda group called for a united front against the war coalition Washington is building.
US jets have been targeting IS fighters in northern Iraq since August 8, and in recent days hit militants southwest of Baghdad for the first time, in a significant expansion of the campaign.
US Central Command said that, in addition to bombing IS fighters threatening the northern city of Arbil, strikes had destroyed a guerrilla ground unit and two supply boats southwest of Baghdad.
The campaign appeared to bear fruit on Tuesday when Kurdish peshmerga fighters -- who now receive Western military supplies and US air support -- retook seven Christian villages overrun by jihadists.
In Washington, Defense Secretary Chuck Hagel told US lawmakers that plans were being laid to hit targets in Syria, where the IS group is holding Western hostages and has a stronghold in the city of Raqa.
"This plan includes targeted actions against ISIL safe havens in Syria, including its command and control, logistics capabilities, and infrastructure," Hagel told the Senate Armed Services Committee.
- 'Close-combat advising' -
Dempsey had gone further than any US official before in saying that the military advisors that Obama has dispatched to bolster Iraqi forces could get drawn into combat.
Obama's administration has insisted that his action against the IS extremists is not the start of another US ground war in the Middle East, and that there will be no large-scale American invasion.
But nearly 300 US military advisors are already working with Iraqi government forces, 300 more are on their way and Dempsey refused to rule out them providing "close-combat advising."
Canada has sent 69 of its soldiers to advise Iraqi forces, Prime Minister Stephen Harper said, and other allies may follow suit.
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"To be clear, if we reach the point where I believe our advisors should accompany Iraqi troops on attacks against specific ISIL targets, I will recommend that to the president," Dempsey said, before his spokesman moved to tamp down expectations.
Dempsey stressed the advisors are "very much in a combat-advisory role" and that there is "no intention" at the moment for them to engage in combat: "I don't see it to be necessary right now."
But he said if there were an "extraordinarily complex" operation planned by Iraqi forces -- such as a bid to recapture the rebel-held city of Mosul -- then advisors could head to the front.
Dempsey had said that any use of US troops in the field would be approved by Obama, explaining: "He told me to come back to him on a case-by-case basis."
- 'Hypothetical scenario' -
White House spokesman Josh Earnest was keen to underscore this, after Dempsey's remarks caused a flurry of speculation that US forces would once again suffer "mission creep" in the Middle East.
Earnest said Dempsey "was referring to a hypothetical scenario in which there might be a future situation in which he might make a tactical recommendation to the president as it relates to the use of ground troops."
"The president has been clear about what that policy is: the president does not believe that it would be in the best interest of our national security to deploy American ground troops in a combat role in Iraq and Syria. That policy has not changed."
Obama has vowed to expand American efforts and US diplomats are scrambling to put together an international coalition for a "relentless" campaign against the IS jihadists, who have overrun large areas of Iraq and Syria.
On Tuesday, Obama met General John Allen, the former Marines commander tasked with coordinating the US-led coalition's mission to "degrade and ultimately destroy" the Islamic State group.
Western nations and 10 Arab countries, including regional powerhouses Saudi Arabia and Egypt, have agreed to back the US-led coalition campaign, but not all will engage in military action.
So far, US forces have carried out 167 air strikes across Iraq. The CIA estimates that the Islamic State organization may be able to field as many as 31,500 fighters -- many of them foreign volunteers.
The slow coming together of this alliance drew a fierce reaction from Al-Qaeda's branches in Yemen and in North Africa, who said jihadist forces must unite against the threat posed by the coalition.
The Islamic State group began as a successor to Al-Qaeda's Iraqi branch, but has escaped from the group's shadow and clashed with its surrogates in Syria, while claiming leadership of global jihad.