Barack Obama has vowed that the United States will not be "dragged back" into military action in Iraq as long as leaders in Baghdad refuse to reform a political system that has left the county vulnerable to a fast-moving Islamic insurgency.
The US president ruled out the possibility of putting American troops on the ground in Iraq, but said he was considering a range of other options drawn up by the Pentagon.
Administration officials said those include strikes using drones or manned aircraft, as well as boosts in surveillance and intelligence gathering, including satellite coverage and other monitoring efforts.
The US, which routinely has an array of ships in the region, has the aircraft carrier USS George HW Bush and an accompanying Navy cruiser in the northern Arabian Sea, while two Navy destroyers from the Bush strike group have been operating in the Persian Gulf.
The ships carry Tomahawk missiles, which could reach Iraq, and the USS George HW Bush is carrying fighter jets that could also easily get to Iraq.
Still, the president appeared to leave himself a clear off-ramp by making military action contingent on a "serious and sincere effort by Iraq's leaders to set aside sectarian differences" between the nation's Sunnis and Shias.
"We can't do it for them," he said. "And in the absence of this type of political effort, short-term military action, including any assistance we might provide, won't succeed."
US intelligence agencies assess that Baghdad is unlikely to fall.. Iraq's Shia soldiers who deserted en masse because they were unwilling to fight and die for Sunni towns such as Tikrit are much more likely to fight for Baghdad and its Shia-dominated national government, US intelligence officials believe. US agencies also assess that the units around Baghdad are marginally better.
Officials said they estimate there are several thousand insurgents, but well short of 10,000.
The security situation in Iraq rapidly deteriorated this week as the al-Qaida-inspired group Islamic State of Iraq and the Levant quickly overran Iraq's second-largest city of Mosul, Saddam Hussein's hometown of Tikrit and smaller communities, as well as military and police bases - often meeting little resistance from state security forces. The militants have vowed to press on to Baghdad.
The rebellion has emerged as the biggest threat to Iraq's stability since the US withdrew its military in late 2011 after more than eight years of war. Mr Obama said the militants also pose a threat to US national security interests, which could ultimately be used as a justification for a unilateral American strike.
Over the past several days, the United States has urged Prime Minister Nouri al-Maliki to make his government more inclusive and avoid further alienating Iraqi Sunnis who are eyeing the insurgency as an alternative to supporting the Shia leadership in Baghdad. That message was delivered to al-Maliki in a phone call from Vice President Joe Biden and also personally by Deputy Assistant Secretary of State Brett McGurk, who has years-long ties to the prime minister and is in Iraq this week to help negotiate a solution
Mr Obama suggested it could take several days to gather the intelligence necessary to make a final decision on the US response to the situation. Following his statement, Mr Obama departed on a four-day trip to North Dakota and California. Officials said he had no plans to cut it short.
For the president, launching military strikes in Iraq would pull the US back into a conflict he declared over more than two years ago. The president has since tried to keep the US out of further conflicts, including in Syria, where a civil war is helping fuel the insurgency in neighbouring Iraq.
Secretary of State John Kerry, travelling in London, said a key difference between striking Syria and taking action in Iraq was the fact that Baghdad was specifically asking Washington for help.
"Under international law, it is clear that when a legitimate nation makes a request for help there is a legal basis for involvement in ways that are different," Mr Kerry said.
Iraqi leaders have been pleading with the US for additional help to combat the insurgency for more than a year. While the U.S. has sold Iraq military equipment, the Obama administration has resisted drone strikes.
Congressional Republicans have accused Obama of ignoring repeated warnings about the worsening conditions on the ground.