The U.S. wants to "take decisive military action" to "check" ISIS in Libya, while still supporting the political process to form a functioning government, Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, told reporters while traveling in Europe.
U.S. military reconnaissance flights over Libya and covert intelligence-gathering missions are now focusing sharply on tracking and locating ISIS's growing presence in that country, according to a senior defense official.
Dunford hopes to develop recommendations in the next several weeks that would offer options for U.S. military action in conjunction with allies such as France, Italy and Britain, the official said.
At the same time, Dunford is working on recommendations for additional U.S. and coalition troops to help the Iraqis retake the city of Mosul in the coming weeks and months.
CNN was not present for the session with reporters, but Dunford's spokesman confirmed the details of his remarks.
The chairman did not specify any U.S. military options. But other defense officials have made clear that they would involve targeting ISIS forces inside Libya, as well as possibly training Libyan forces outside the country to go back in and fight the group.
Concern has grown in recent weeks as the latest intelligence assessments show the number of ISIS operatives inside Libya could now number in the low thousands. The U.S. also believes some senior ISIS leaders have gone there.
ISIS fighters are going to Libya when they cannot get into Syria and Iraq, but some are also leaving those areas in the face of ongoing airstrikes and fighting against them in place like Raqqa, ISIS' self-declared capital in Syria. The U.S. also is concerned that ISIS will make a move against Libyan oil installations in order to restore cash flow it has lost in U.S. bombings.
In November, the first U.S. airstrike against ISIS in Libya killed Abu Nabil, a senior operative. Now, however, the U.S. is privately pressing allies to also take more action against ISIS in Libya.
By getting better intelligence on where ISIS is located in Libya and acting against the group, the U.S. hopes to put a "firewall" around its presence inside the country so it cannot expand further into Africa or move toward southern Europe, the defense official said.
Dunford's remarks came as another top U.S. military leader weighed in on ISIS in Libya.
"There is concern about Libya," said Gen. Joe Votel, head of U.S. Special Operations Command. "It can't all be about Iraq and Syria."
Votel has been nominated to take over Central Command, which is running the U.S.-led campaign in Iraq and Syria.