Secretary of State John Kerry has called for a global coalition to combat the Islamic State jihadist group and their "genocidal agenda".

His remarks in an opinion piece published by  The New York Times 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 their advances through Iraq and Syria.

In Britain, meanwhile, authorities raised the terror alert level over fears of possible jihadist attacks.

And in Syria, clashes broke out between Philippine UN peacekeepers and another jihadist group, the Al-Qaeda-linked Al-Nusra Front, which is also holding hostage dozens of Fijian UN peacekeepers.

Writing in the 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, [so] the cancer of ISIS will not be allowed to spread to other countries"

He said he and Defense 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".

US President Barack 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 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. 

The Islamic State (ISIS) 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.

The government of embattled Syrian 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 rights violations including the use of chemical weapons.

The group's progress has also raised regional fears, with Saudi King Abdullah warning Saturday that "they will reach Europe in a month and America in another month," if left unchecked.

"It is no secret to you, what they have done and what they have yet to do," he was quoted as telling incoming ambassadors at a welcoming ceremony.

"I ask you to transmit this message to your leaders: 'Fight terrorism with force, reason and (necessary) speed'."

Britain on Friday said it was raising its terror alert level to "severe," meaning an attack is "highly likely," though Washington said it had no plans to follow suit.

The threat posed by ISIS is just the latest fallout from Syria's conflict, which began in March 2011 as a peaceful uprising against Assad. 

On Wednesday, fighters from another jihadist group in Syria, Al-Nusra Front, seized the Syrian side of the Quneitra border crossing with the Israeli-occupied Golan Heights.

They took hostage 44 Fijian UN peacekeepers and have surrounded a group of more than 70 other UN peacekeepers from the Philippines.

On Saturday, Philippine Defense minister Voltaire Gazmin said some troops had been "extricated" from one of their two positions but added that another group of Filipino soldiers "is now under attack".

He said the firefight began early Saturday morning, but that all troops were safe.

The violence in Syria began in March 2011, with an uprising against Assad that spiraled into a bloody conflict after a government crackdown.

More than 190,000 people have been killed, according to the UN, and the official number of refugees from the conflict hit three million on Friday.

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 Organization 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 ISIS swept through parts of five Iraqi provinces in an advance that began in June.