Representatives from 23 countries met in Rome on Tuesday to discuss the growing threat from IS in Libya.
IS took control of Sirte, the hometown of former Libyan leader Muammar Gaddafi, last year.
Disagreements between rival administrations in the country have hampered efforts to fight IS.
The Islamist group is believed to have received support from some loyalists of the former regime.
But Ismail Shukri, the head of intelligence in the city of Misrata, told Newsnight there had been an influx of foreign fighters in recent months.
"The majority [of IS fighters in Sirte] are foreigners, around 70%. Most of them are Tunisians, followed by Egyptians, Sudanese and a few Algerians.
"Add to that the Iraqis and the Syrians. Most of the Iraqis come from Saddam Hussein`s disbanded army."
Mr Shukri said senior IS commanders were taking refuge in Libya, under pressure from international airstrikes in Iraq and Syria.
"Some of their members, especially those with long-term importance to IS, are taking refuge here. They view Libya as a safe haven."
Authorities in Misrata say they are preparing an offensive against Islamic State militants in Sirte.
But in the town of Abugrein, 120km (75 miles) south of Misrata, the BBC saw little evidence of an imminent confrontation.
Abugrein represents the final line of defence against IS. Beyond that, IS controls the road east.
Commanders in Abugrein told Newsnight that their forces, loyal to the government in Tripoli, numbered around 1,400 - less than half the estimated strength of IS.
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