Reuters An armed motorcade belonging to members of Derna's Islamic Youth Council, consisting of former members of militias from the town of Derna, drive along a road in Derna, eastern Libya October 3, 2014.
Talking to reporters on Wednesday, Pentagon Press Secretary Peter Cook, said the US was "looking at military options," as military personnel gather information about the situation on the ground.
A security vacuum has persisted in the oil-rich country since the capture and death five years ago of dictator Muammar Gaddafi, who had ruled the country for more than 40 years. The US air campaign that helped topple Gaddafi ended after the his death in 2011.
Numerous attempts to form a unity government between opposition leaders failed and the ensuing vacuum created the opporunity for ISIS (also known as the Islamic State, ISIL, or Daesh) to expand its operations and take control of large swathes of territory.
In recent weeks, ISIS has also been targetting Libya's very large oil industry (400,000 barrels a day, down from 1.6 million barrels a day when Gaddafi was in power).
Cook ackowledged that presently, the US was continuing to assess the threat presented by ISIS, "We are extremely worried about the metastasis of ISIL to other locations," he said, adding that Libya was "a place where we've seen movement of ISIL forces."
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"The reason for the presence of those troops is to, again, get a sense of the forces on the ground, the players on the ground, and exactly what's happening, because it is a muddled picture right now," Cook explained.
Cook said the US was continuing to "support the effort to forge a government in Libya," stressing how important this was to the future of the country. After the civil war in 2011, the National Transitional Council of Libya was established as the de facto government in Libya and passed on its powers to the General National Congress in 2012.
Since then, the country has failed to establish a unity government that includes both groups fighting for power.