"The current military operation and advance on Tripoli are increasing the suffering of the Libyan people and putting civilian lives at risk."
Libya has been riven by divisions since the NATO-backed overthrow of dictator Moamer Kadhafi in 2011, with various armed groups and two parallel governments vying for territory and oil wealth.
Heavy arms fire was heard during much of the night on the southeastern outskirts of Tripoli as strongman Khalifa Haftar's forces pressed an assault aimed at taking the capital from the UN-backed Government of National Accord (GNA).
Haftar's self-styled Libyan National Army (LNA), which controls swathes of the country's east, appears to be advancing on two fronts, from the south and southeast of Tripoli, while coastal roads to the east and west of the city are defended by fighters loyal to the GNA.
Stoltenberg insisted "there is no military solution to the situation in Libya" and called on all parties to pursue the political path instead.
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