Mohammad al-Amari, a minister of state in the Government of National Accord, signed documents passing control of the key foreign ministry complex in the north of the capital to the new government, an AFP journalist said.
The GNA also Monday took the ministries of planning and religious endowment.
Over the past week, the transport, social affairs, environment, and youth and sports ministries have likewise been transferred to the new government.
The international community sees the GNA as the best hope for oil-rich Libya, which has been roiled by turmoil since the 2011 ouster and killing of longtime ruler Moamer Kadhafi.
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Libya's State Council, the most senior consultative body created under a UN-brokered power-sharing deal in December, met for the first time on Friday.
The GNA has still not been endorsed by a vote of confidence from the internationally recognised parliament now sitting in Tobruk in the country's far east.
However, on Thursday 102 MPs of the Tobruk parliament -- a majority -- expressed confidence in the new government despite the lack of a formal vote.
Libya's warring rivals have come under intense international pressure to rally behind the unity government at a time when the country is grappling with a growing jihadist threat.
While the GNA has now having taken control of several ministries, these have yet to get down to work.
"As soon as the ministers take the oath of office, they can begin working from these ministries," Amari said.