U.S. Defense Secretary Ash Carter warned of the threat from Islamic State in Afghanistan during a surprise visit days after the Pentagon painted a grim picture of worsening security there, Reuters reports.

Carter met troops at a U.S. base near the city of Jalalabad in Nangarhar Province, where about 600 U.S. troops are stationed. Operating Base Fenty is a hub for training, logistics and counter-terrorism efforts across eastern Afghanistan.

"We are seeing little nests of ISIL (Islamic State) spring up around the world, including here in Afghanistan ... but I will say that that is a threat that we track very closely," Carter told a news conference after meeting acting Afghan Defense Minister Masoom Stanekzai on Friday, December 18.

U.S. Army General John Campbell, who leads international forces in Afghanistan, said Islamic State had coalesced over the last five or six months in Nangarhar and Kunar provinces and had been fighting the Taliban for several months.

He estimated there were between 1,000 and 3,000 members of Islamic State in Afghanistan and said its presence had forced the Taliban to re-direct some resources away from fighting Afghan security forces.

"It doesn't have the capability, I believe, to go to Europe and attack Europe and go to our homeland at this point," Campbell said.

"Left unchecked it will. They’ve said they want to do that."

In a report to the U.S. Congress released this week, the Pentagon said that from the beginning of the year to mid-November, there were 27 percent more high-profile attacks in the capital Kabul compared with the same period last year.