US lawmakers of both the Democratic and Republican parties said they backed Obama's decision to authorize airstrikes against Iraqi militants.
UK Prime Minister David Cameron said in a statement that he welcomed Obama's decision to accept the Iraqi government's request for help; but ruled out participation in any military operations.
He said British defence secretary Michael Fallon will chair a meeting of the UK's COBRA emergency committee this morning to ''establish what more we can do to provide help to those affected''.
President Obama, in a televised speech late on Thursday, said that at the request of the Iraqi government, American planes had already carried out airdrops of food and water to tens of thousands of Iraqi religious minorities atop a mountain surrounded by militants and desperately in need of supplies.
The Yazidis, who follow an ancient religion with ties to Zoroastrianism, fled their homes after the ISIS, as the militants are generally known, issued an ultimatum to convert to Islam, pay a religious fine, flee their homes or face death.
Obama said he had authorised limited airstrikes against Sunni militants in Iraq, potentially re-engaging the US military in a conflict that he had pledged to quickly pull out of when he first came to office.
Obama said that the strikes, if needed, would be used to protect US personnel and the Yezidis, a minority sect concentrated in northern Iraq, who have been targeted by militants and are stranded on a mountain.
Obama spoke following a day of urgent discussions with his national security team. He addressed the nation only after the American military aircraft delivering food and water to the Iraqis had safely left the drop site in northern Iraq.
The Iraq extremists ''have called for the systematic destruction of the entire Yezidi people, which would constitute genocide'', Obama said last night in remarks from the White House. ''The United States of America cannot turn a blind eye.''
Crude oil rallied following Obama's announcement. Brent for September settlement gained as much as $1.41 to $106.85 a barrel on the London-based ICE Futures (DJA) Europe exchange and was at $106.49 at 2:36 p.m. Singapore time. Prices are up 1.6 percent this week, the most in two months.
Obama said the US must also protect its diplomats and civilians if the militants move toward the Kurdish city of Erbil, the location of an American diplomatic outpost.
The president's airstrike authorisation doesn't cover other areas where the Islamic State militants are encroaching, including Syria and Lebanon, the officials said.
''I have been careful to resist calls to turn time and again to our military,'' Obama said. ''But when the lives of American citizens are at risk, we will take action.''
The escalation in US involvement in Iraq comes as the Islamic State, the extremist group that's conquered swaths of northern Iraq since June, extended its advance by seizing the Mosul Dam, the country's largest. The Sunni militants' offensive, and their threats to kill religious minorities, has panicked tens of thousands of people and emptied towns that for centuries have been home to Yezidi and Christian communities.