President Barack Obama has authorised air strikes against IS, a Sunni Muslim group formerly known as Isis, that has been gaining ground in northern Iraq since it launched its onslaught in June, if they threaten US interests or to prevent the slaughter of religious minorities.
However, the president said US troops would be not be sent back to Iraq.
The US has earlier made humanitarian air drops to Iraqis under threat from Islamic State (IS) militants.
IS has seized Qaraqosh, Iraq‘s biggest Christian town, prompting residents to flee.
President Obama, while speaking at the White House on Thursday evening after meetings with his national security advisers, said US military aircraft had dropped food and water to members of the Yazidi religious minority community who were trapped on Mount Sinjar by the IS fighters.
Officials had warned that the Yazidis faced starvation and dehydration if they remained on the mountain, and slaughter at the hands of the IS if they fled.
“The US cannot and should not intervene every time there is a crisis in the world,” the President said.
He, however, said the US could not turn a “blind eye” to the prospect of violence “on a horrific scale”, especially when the Iraqi government had requested assistance.
“We can act, carefully and responsibly, to prevent a potential act of genocide,” he went on. “Today America is coming to help.”
He said he had authorised air strikes at IS fighters, should the militants’ convoys move toward Irbil, where there is a significant presence of US diplomats and military advisers, or threaten Baghdad.
In addition, he authorised strikes “if necessary” to help Iraqi government forces break the siege at Mount Sinjar and rescue the trapped civilians.
He added that the US could and should support moderate forces that could bring stability to Iraq, and he said there was no “American solution” to the turmoil plaguing Iraq.
“The only lasting solution is reconciliation among Iraqi communities and stronger Iraqi security forces,” he said.
The president spoke hours after the UN Security Council met to discuss the situation.
“The members of the Security Council call on the international community to support the government and people of Iraq and to do all it can to help alleviate the suffering of the population affected by the current conflict in Iraq,” said UK Ambassador to the UN Mark Lyall Grant.
UN Secretary-General Ban Ki-moon said he was “deeply appalled”.
Meanwhile, the US has warned that the situation for Iraq’s minority groups is threatening to become a “humanitarian catastrophe”.
IS says it has created an Islamic state in the territory it controls.