Responding to a UN Security Council plea for the international community to help Iraq cope with a flood of refugees prompted by a jihadist offensive, US President Obama ordered the first US air strikes on Iraq since the end of Washington's occupation in 2011.
US forces bombed the jihadists after they shelled Kurdish regional government forces defending their capital Arbil.
In addition to the military action, a US defence official also confirmed that planes had already dropped "critical meals and water for thousands of Iraqi citizens" and vowed further drops if needed.
But just hours after Obama pledged potential military strikes, Britain, which joined the US-led invasion of Iraq in 2003, moved swiftly to rule out military intervention of its own.
Prime Minister David Cameron stressed that he "fully agreed" with Obama that "we should stand up for the values we believe in -- the right to freedom and dignity, whatever your religious beliefs." Nevertheless, a Downing Street spokesman emphasised: "We are not planning a military intervention."
After an emergency government meeting to discuss the situation in northern Iraq, Defence Secretary Michael Fallon said: "What we have decided today is to assist the United States in the humanitarian operations that started yesterday. We are offering technical assistance ... in terms of refuelling and surveillance."
"We are offering aid of our own which we hope to drop over the next couple of days in support of the American relief effort, particularly to help the plight of those who are trapped on the mountain," added Fallon.
France, which opposed the 2003 invasion, welcomed the "important decision" Obama had made to launch air strikes and vowed "support" for those fighting the militant advance in Iraq but without specifying what form that might take.
"France will examine with the United States and all its partners what actions can be taken to jointly offer the necessary support to end the suffering of the civilian population," President Francois Hollande said in a statement that did not directly refer to the bombing. France is "ready to play a full part" in this support, added Hollande, without offering further details.
Germany, which also opposed the 2003 war, vowed to boost its humanitarian aid by 2.9 million euros ($3.9 million) and pledged more help if needed. "It is clear that is not enough and we have to see what we can do beyond that," Foreign Minister Frank-Walter Steinmeier said in a statement.
"The killing, systematic forced displacement and forced conversion of Christians, Yazidis and members of other minorities by... terrorists in Iraq represent a new dimension of the horror," Steinmeier said. "We condemn these despicable crimes, targeted at entire communities, in the strongest terms."
However, as Western governments condemned the horror, charities and experts warned that the response was too little, too late.
British charity Save the Children said it had never seen such a rapid displacement of people as in the past two months in Iraq, with 1.2 million people fleeing their homes, and called for unfettered humanitarian access to the worst affected areas.
The charity's country director, Tina Yu, said: "We're seeing children and families who've fled their homes, often in the middle of the night, fearing for their lives and with nothing but the clothes on their backs."
Jean-Charles Brisard, an independent terrorism consultant, said France's response in particular had been "cosmetic". "France and its European partners have made declarations of intent but there has been no action," he told AFP. "We're now trapped because we've waited too long against very well-structured groups."