US Secretary of State John Kerry has called for a global coalition to combat Islamic State fighters' "genocidal agenda" after President Barack Obama admitted he had no strategy to tackle the group.
Kerry's call followed a decision by Britain to raise its terror alert level over fears of possible jihadist attacks.
The number of refugees who have fled Syria, where rebels have been battling the regime as well as jihadists, has now exceeded three million people, the United Nations said.
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 with European counterparts on the sidelines of the summit to enlist assistance, and then travel on to the Middle East to build support "among the countries that are most directly threatened."
US President Obama has acknowledged that Washington has no strategy as yet to tackle the Islamic State, which has declared an Islamic "caliphate" in large swathes of territory under its control in Iraq and Syria.
But Kerry said in his op-ed on Friday that the United States would be putting forward an action plan at a summit meeting of the UN Security Council in September, when Washington will hold the group's rotating presidency.
"What's needed to confront its nihilistic vision and genocidal agenda is a global coalition using political, humanitarian, economic, law enforcement and intelligence tools to support military force," Kerry said.
US bombing IS in Iraq
The Islamic State (IS) has prompted widespread concern as it advances in both Syria and Iraq, killing hundreds of people, including in gruesome beheadings and mass executions.
The United States began carrying out air strikes against the group in Iraq earlier this month, but has yet to decide if it will expand that military action into Syria.
There, the government of embattled President Bashar al-Assad has said it is willing to cooperate on tackling jihadists, but that any military action on its soil must be coordinated.
That is a tough sell for Washington, which has long backed the rebels seeking Assad's overthrow and accuses his regime of serious rights abuses, including the use of chemical weapons.
The threat posed by IS, which has attracted hundreds of Western volunteers to its ranks, prompted Britain to raise its terror alert level on Friday.
The move takes the alert level to "severe," meaning an attack is "highly likely."
British Prime Minister David Cameron told reporters there was "no doubt in my mind" that jihadists from IS had their sights set on targets in Europe.
But the White House said Washington had no plans to follow suit though US national security officials had been in close contact with London on the issue.
3 million Syrian refugees
The threat posed by IS is just the latest fallout from Syria's conflict, which began in March 2011 as a peaceful uprising against Assad.
The violence there has now driven three million Syrians to flee abroad, creating the "biggest humanitarian emergency of our era," UN refugee agency chief Antonio Guterres said Friday as the new milestone figure was announced.
The war has also killed more than 190,000 people according to the UN, and seen jihadist groups like IS and Al-Qaeda's Syrian affiliate Al-Nusra Front gain new ground and prominence.
Al-Nusra is holding at least 44 UN peacekeepers in the Golan Heights, after seizing the Syrian side of the Quneitra border crossing with the Israeli-occuped Golan.
The 44 troops, from Fiji, "are safe and in good health" a UN spokesman said Friday without saying whether the men had been freed yet.
Another 72 peacekeepers from the Philippines have been surrounded by rebels, according to the UN, which said they "have not been harmed and are in good health."
The peacekeepers are from the UN Disengagement Observer Force (UNDOF) tasked with monitoring a ceasefire between Syria and Israel since 1974.
In Iraq meanwhile, government forces readied an offensive to break a two-month IS siege of the Shiite Turkmen town of Amerli.
According to the International Organisation for Migration, 1.6 million Iraqis have been displaced this year, with more than 850,000 leaving their homes this month alone.
Many of those were forced to flee as IS swept through parts of five Iraqi provinces in an advance that began in June.