The UN mission in Libya, UNSMIL, last week reported grave human rights violations during the clashes between mostly Islamist fighters and a nationalist militia.
On August 22, Fajr Libya (Libya Dawn), a mainly Islamist alliance, seized Tripoli airport from the nationalists. Three days later, Libya's embattled government said Tripoli was in the hands of armed militias.
"Libyan militia forces battling for control of Tripoli and surrounding areas have engaged in attacks on civilians and civilian property that in some cases amount to war crimes," said Human Rights Watch.
"The militias have seized people and looted, burned, and otherwise destroyed property," said the New York-based HRW.
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The watchdog said it documented attacks by both sides but that since Libya Dawn seized the airport, "its forces have committed further violations."
They included attacks on journalists, government officials and ordinary citizens suspected of siding with the nationalist forces, said HRW.
Prime Minister Abdullah al-Thani has accused Libya Dawn fighters of ransacking and torching his home in southern Tripoli as well as the residence of Transport Minister Abdelkader Mohamed Ahmed.
"All warring parties, as well as the Libyan government, should respect their obligation to protect civilians at all times and to hold their forces accountable when they commit crimes," said HRW's Sarah Leah Whitson.
The United Nations estimates that the Tripoli fighting has displaced at least 100,000 people, and another 150,000 have fled the country, including many migrant workers.