WASHINGTON/KIRKUK, Iraq: The United States is not aware of any specific threat to the U.S. homeland from ISIS militants, the Department of Homeland Security said Friday, after Britain raised its international terrorism threat level.

 

ISIS militants and their supporters, however, “have demonstrated the intent and capability to target American citizens overseas,” Homeland Security Secretary Jeh Johnson said in a statement.

 

He noted that the DHS had taken steps over the summer to strengthen security at overseas airports with direct flights to the United States.

 

Johnson said that he had spoken to U.K. Home Secretary Theresa May about Britain’s decision to raise its terrorism alert to the second-highest level.

 

It is the first time since mid-2011 that Britain has been placed on this high of an alert level.

 

May said Friday that Britain had increased its threat level because militant groups in Syria and Iraq were “planning attacks against the West” and some attack plots were “likely” to involve fighters from Britain and elsewhere in western Europe.

 

However, U.K. authorities have also said they have no knowledge of any “imminent” plot to attack British targets.

 

Speaking just after the terrorism threat level was raised, British Prime Minister David Cameron pledged to plug gaps in Britain’s armory to combat terror, describing the extremist threat posed by ISIS as being more dangerous than even that of Al-Qaeda.

 

“What we are facing in Iraq now with ISIS is a greater threat to our security than we have seen before,” Cameron said.

 

He told reporters that while the Taliban had facilitated Al-Qaeda terrorism, ISIS was “effectively a state run by terrorists.”

 

He said the ambition to create an Islamist caliphate was not something that could be ignored.

 

“We could be facing a terrorist state on the shores of the Mediterranean and bordering a NATO member,” he said, referring to Turkey.

 

On the ground, Iraqi aircraft carried out strikes targeting jihadists around the town of Amerli.

 

Security forces are preparing a major assault to break through to the besieged Shiite Turkmen town, officers said.

 

Amerli has been surrounded by ISIS for more than two months, and its residents face severe shortages of food and water, as well as the threat from the militants.

 

Residents of Amerli are in grave danger both because of their Shiite faith, which the jihadists consider heresy, and their resistance to the militants, which has drawn harsh retribution elsewhere.

 

Iraqi forces retook several villages on the way to Amerli Thursday, as part of a major operation to free the town.

 

And officials say that Washington is weighing up the possibilities of both aid drops and airstrikes to help the town.

 

“It could be a humanitarian operation. It could be a military operation. It could be both,” a U.S. defense official said, speaking on condition of anonymity.

 

United Nations Secretary General Ban Ki-moon slammed “brutal killings of civilians” by ISIS in northern Iraq, saying Friday that the Sunni radical group was tearing apart whole communities.

 

Ban told a United Nations conference on the Indonesian resort island of Bali: “All major faiths value peace and tolerance.

 

“That is why I am especially outraged by the reports from Iraq of brutal killings of civilians by ISIS.

 

“Whole communities that had lived for generations in northern Iraq are being forced to flee or face death just for their religious beliefs.”

 

Ban said that communities should not be threatened simply because of “who they are and what they believe.”

 

ISIS posted a video Friday of the execution of a captured Kurdish fighter, in a warning to Iraqi Kurdish leaders to end military cooperation with Washington, a monitoring group said.

 

The United States has carried out a wave of airstrikes against the jihadists in northern Iraq, helping Kurdish forces to claw back ground that was lost to the militants earlier this month.

 

The video, entitled “A message in blood to the leaders of the American-Kurdish alliance,” opens with 15 men in orange jumpsuits standing around the ISIS flag.

 

Three of the men ask Massud Barzani – the Kurdish regional president – “and the Kurdish government to end their relationship with the U.S. ... military intervention in northern Iraq.”

 

The video comes hot on the heels of another released by ISIS, showing the beheading of American journalist James Foley and threatening another kidnapped reporter Steven Sotloff with the same fate if U.S. airstrikes are not halted.