Warning comes as diplomats from 23 nations meet in Rome to plan steps to stop group from seizing North African country.
ISIL fighters have streamed into Libya in recent months, heightening fears the fighters are gaining ground and influence in the divided north African country.
About 5,000 Islamic State of Iraq and the Levant fighters are now in Libya – double earlier estimates – while the number in Iraq and Syria has dropped, a security analyst said on Thursday.
The updated figures come as the US administration of President Barack Obama faces growing calls for the American military to step up action against ISIL in Libya, where fighters have already seized the city of Sirte and an adjoining length of Mediterranean coastline.
Patrick Skinner, a former CIA case officer now with The Soufan Group consultancy, said the US has few good Libya options, but there is growing consensus something must be done.
“They are just terrified of it getting much worse, fast,” Skinner told the AFP news agency.
“Once [ISIL] takes something, it’s really hard and really bloody and really expensive to take it back.”
NATO defence ministers are meeting in Brussels next week to evaluate the ongoing US-led campaign against ISIL and to discuss ways of redoubling efforts.
No large-scale US military action is contemplated in Libya, senior US administration officials told the Associated Press news agency.
Military options under consideration include raids and advisory missions by US special operations forces and air strikes, the officials said on condition of anonymity.
“The last thing in the world you want is a false caliphate with access to billions of dollars of oil revenue,” John Kerry, the US secretary of state, said on Tuesday.
Meanwhile, the US now says that there are between 19,000 and 25,000 ISIL fighters in Iraq and Syria – down from a previous count of about 30,000.
But there are growing fears about the fate of Libya, which has been in chaos since the NATO-backed ousting of long-time dictator Muammar Gaddafi in 2011, thanks largely to its air power. Since 2014, Libya has been split between two rival authorities, each backed by different militias and tribes.
Skinner pointed to the irony that Western leaders are now scrambling for solutions in Libya.
“The international coalition is going to air strike its way out of the chaos created by air strikes,” he said. “That’s actually what people are considering. Something has to be done. The horrible reality is: what is that something?”
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