The United States is not aware of any specific threat to the US homeland from Islamic State militants, the Department of Homeland Security said on Friday after Britain raised its international terrorism threat level.Islamic State 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 DHS took steps over the summer to strengthen security at overseas airports with direct flights to the United States.Johnson said he has spoken to UK 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 on Friday that Britain increased its threat level because militant groups in Syria in Iraq were "planning attacks against the West" and some attack plots were "likely" to involve foreign fighters from Britain and elsewhere in western Europe.However, UK authorities also have said they have no knowledge of any "imminent" plot to attack British targets.US and European officials have said that because of relaxed border controls between European Union countries, it is difficult to track travel to Syria and Iraq by would-be foreign fighters. Often suspected militants are not identified until after they return to their home countries.US authorities are particularly concerned about former foreign fighters in Syria and Iraq who have British or other European passports that would allow them to enter the United States with instant visas and minimal security vetting.US and European officials have estimated that as many as 100 Americans have traveled to Syria or Iraq to fight with militants, along with four or five times as many Britons and hundreds of other Western Europeans."This is a threat that the United States has been focused on. We've been coordinating closely with our allies, both the Brits, but others in Europe, about countering this threat and mitigating it," White House spokesman Josh Earnest said at a news briefing.There was no plan to raise the US threat assessment level, he said.While the United States once characterized threats under a system of color coded warnings, the Obama administration abandoned that system and now issues warnings targeted to particular transport or economic sectors.