"The situation is extremely serious," Mohamed al-Dayri, foreign minister for Libya's internationally-recognised government based in Misrata, told AFP on a visit to Paris.
"People are dying, are crucified, are disinterred from their graves, are burned alive. Libyans don't understand why the international community doesn't wake up to these dangers."
Libya has two rival governments and has been torn apart since the international community helped oust its leader Moamer Kadhafi in 2011.
Dayri said the Islamic State (IS) group was present in the towns of Derna, Benghazi, Sirte and Sabratha.
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"They have not yet seized oil fields, but we fear they might come to control several wells," he added.
"On Saturday, there was a call from their leaders in Iraq and Syria to reinforce their ranks in Libya. They want to make Libya a rear base."
He said IS currently had limited means in Libya and had faced pushback from local populations working alongside other armed Islamist groups including Al-Qaeda, notably in Derna in June.
He called for the United Nations to lift the arms embargo imposed in 2011.
"We are not talking about sophisticated military equipment, but we need the minimum to fight terrorism in an adequate manner," he said.
"We also hope for an (international) intervention as soon as possible because the danger is growing. But not troops on the ground. We are hoping for aerial support for the Libyan armed forces on the ground.
"The international community waited for Mosul to fall before intervening in Iraq. We don't want to see Tripoli or Misrata fall," he said.