WASHINGTON - Two and a half years after President Barack Obama disentangled America from a long, unpopular war in Iraq, his options for helping the Iraqi government stave off a militant onslaught are slim as doubts simmer over whether even punishing air strikes would be effective.

He will announce in coming days how far he is willing to go in responding to the crisis in Iraq, where militants are sweeping south towards the capital Baghdad in a campaign to recreate a large mediaeval Islamic caliphate spanning Iraq and Syria.

While Obama has ruled out sending combat troops, US officials say options under consideration include air strikes on Sunni insurgents threatening the Shi'ite-led government, accelerated delivery of weapons and expanded training of Iraqi security forces. The US already has increased intelligence-gathering flights by drone aircraft over Iraq, officials said.

There is growing skepticism both inside and outside of the administration whether Washington has the will, let alone the power, to halt Iraq's slide into a civil war that could tear it apart. The collapse of Iraq's US-trained army in the north this week has compounded concerns that fast-moving events are unfolding beyond America's ability to control them, say officials.

"It is a colossal mess," said one senior US official.

Hoping to mitigate the risk of a failed US response, the administration may opt for a phased approach, first trying to shore up Iraqi forces and possibly resorting to more direct military action if the situation deteriorates further, according to a source familiar with the White House's thinking.

The biggest questions center on whether the United States will carry out air strikes, either with warplanes or unmanned drones, against militants of the Islamic State in Iraq and the Levant, or ISIL, which moved swiftly to seize the northern cities of Mosul and Tikrit this week and now threaten Baghdad.

Such attacks, an option the Pentagon described on Friday as "kinetic strikes", could be launched from aircraft carriers or from the sprawling US air base at Incirlik in Turkey. The carrier USS George H.W. Bush and its strike group are already "in the region," the Pentagon said on Friday.

While a US air assault could send a tough message to ISIL forces about Washington's commitment to the survival of the besieged Iraqi government, national security officials are raising concerns about the US ability to target roving bands of insurgents and seriously damage their fighting capabilities.

Air strikes that damage cities or Iraqi infrastructure could worsen the crisis, said two US national security sources. Another big concern is the risk of hitting the wrong people.

"TARGETED" AND "PRECISE"

Obama's insistence on Friday that any military action would be "targeted" and "precise" appears to reflect a desire for a cautious course that avoids civilian casualties and prevents war-weary Americans from being dragged back into Iraq's sectarian quagmire.

A former US official with knowledge of the situation said that, in discussions within the administration, the White House is seeking to limit the extent of American military involvement, casting doubt over whether the White House would go ahead with a Pentagon-proposed package of military equipment, training and potential air strikes.

The former official, who requested anonymity to discuss internal government deliberations, said Obama and his top aides were focused on increased military sales to the government of Prime Minister Nuri al-Maliki and leery of proposals for drone strikes against ISIL.

The idea of speeding up delivery of US weapons to Iraqi forces is also not without drawbacks.

While shipments of small arms and counterterrorism equipment

may be possible in the near term, large military hardware such as F-16 jet fighters and Apache attack helicopters take much more time to move out of the production pipeline.

Transfer of more of the Hellfire air-to-ground missiles that Iraq has requested could be accelerated. Lockheed Martin Corp , which makes the Hellfire, said it would work with the US government to step up those deliveries if asked.

But US officials may be wary of moving too quickly in this area, especially after seeing US-supplied equipment such as Humvee patrol vehicles and artillery fall into militants' hands during their lightning advance this week.

The Pentagon had pushed for months, sometimes against resistance from White House policymakers, for Iraq to be given a package of enhanced military support to combat the insurgency. But some analysts say proposals on the table are insufficient to help Iraqi forces turn the tide against advancing militants.

"They (the administration) have to do something," said former CIA and White House official Ken Pollack, who is now at the Brookings Institution think tank.

But he said the most recent US proposals amounted to mostly a counter-terrorism package "which will basically have no impact on the situation."

And he suggested it could even further complicate matters by furthering the perception that the United States is squarely on the side of Iraq's Shi'ite government, which has alienated large swaths of the country's Sunni minority.

Obama's deliberations on the possible use of military force in Iraq echoed last year's debate on whether to strike Syria over the use of chemical weapons.

The president has again promised to "consult with Congress" but he stopped short of saying he would bring the issue to a vote by lawmakers. Congressional opposition to the Syria strike plan contributed to Obama's decision not to go ahead with it.