US Secretary of State John Kerry has called for a global coalition to combat Islamic State militants and their "genocidal agenda".
His remarks came as Saudi Arabia's King Abdullah warned that the West would be the group's next target unless there was swift action to stem its advances through Iraq and Syria.
Australia, meanwhile, joined Britain in heightening warnings against terrorism, with both governments concerned that their citizens who have joined Islamic State may return with plans to attack on their home soil.
Writing in The New York Times, a week before a Nato summit in Wales, Kerry urged "a united response led by the United States and the broadest possible coalition of nations".
He said he and Defence Secretary Chuck Hagel would meet European counterparts on the sidelines of the summit to enlist assistance, and then travel to the Middle East to build support "among the countries that are most directly threatened".
The Saudi king's comments also appeared aimed at drawing Washington and Nato forces into a wider fight against extremists in the region. Saudi Arabia has backed rebels fighting in Syria, but is concerned that extremists there could turn their weapons on the kingdom.
"If we ignore them, I am sure they will reach Europe in a month and America in another month," he told foreign ambassadors.
White House Press Secretary Josh Earnest said there were no plans to raise the threat level in the US. The administration had been in contact with counterparts in the UK about the British decision, he said.
US Homeland Security Secretary Jeh Johnson said that the department took steps over the summer to strengthen security at overseas airports with direct flights to the United States.
Australia's government has assessed there are about 60 of its citizens fighting in Syria and Iraq for terrorist groups, with about 100 more involved in facilitation and support for those groups.
Australia's terrorism public alert system remained at "medium", indicating that an attack could occur.
Additional reporting by Reuters