Libya has been a springboard for illegal migration to Europe, especially Italy, since becoming engulfed in chaos following a 2011 revolution that toppled and killed its longtime ruler Colonel Moamer Kadhafi.
Tens of thousands of migrants, mostly from sub-Saharan Africa, have crossed the Mediterranean Sea to Europe each year on overcrowded and flimsy boats run by unscrupulous people traffickers.
Kacem said the latest rescue operations took place on Monday and yesterday off the coasts of Sabratha and Sidi Bilal, ports west of Tripoli.
A total of 290 migrants, all African, were rescued in two other operations off Sidi Bilal, he added.
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A total of 125,000 migrants have so far crossed the Mediterranean by boat this year while an estimated 2,400 have died en route, according to UN figures.
The vast majority of arrivals have landed in Italy, which lies a mere 300 kilometres (186 miles) from Libyan shores, before travelling on to other EU members.
Italy has sent naval vessels at the request of Libya's UN-backed Government of National Accord to assist Tripoli in intercepting migrants, on a mission disputed by rival authorities in eastern Libya.