Colonel Muncif al Walda, a senior police officer in the nearby city of Misrata, said: "Illegal immigration is a menace because it brings and encourages foreign fighters to come and fight with Isis.
"Most of the migrants want to go to Europe, but some want to link up with Isis.
"Unfortunately, here in Libya we are right in the middle of the migration rat run."
Libyan officials spoke out as Britain and America increased their pressure on Libya's new Government to accept Western military help in tackling Isis, including 6000 Western troops on the ground in training roles.
Since establishing a foothold in Sirte a year ago, Isis is believed to have built up an army of between 2000 and 3000 fighters, turning the port city into a Libyan version of the group's Syrian stronghold of Raqqah.