Human Rights Watch on Thursday denounced a series of attacks against shrines of the mystical Sufi branch of Sunni Islam in Libya by "extremist" militias.

"Sufi religious sites are under assault in Libya," the New York-based rights watchdog said after two attacks in the capital Tripoli over the past two months.

It said unidentified assailants torched the Zawiyat Sheikha Radiya, a historic Sufi mosque in Tripoli, heavily damaging it on November 28, on the eve of a feast marking the birthday of Prophet Mohammed.

Another attack on October 20 destroyed the Sidi Abu Gharara mosque, also in Tripoli, said HRW.

The report comes after suspected Islamic State group jihadists killed more than 300 people at a Sufi mosque in Egypt's restive North Sinai late last month.

In much of the Muslim world, Sufism has for centuries been accepted and practised by mainstream Muslims and Sunni Islam's most important theologians.

But Salafists, or hardline Islamists, view Sufis as heretics.

HRW said Sufi shrines have come under attack in Libya since the 2011 uprising that toppled and killed dictator Moamer Kadhafi.

After the revolt the country descended into chaos with rival militias and administrations vying for power.

"Successive interim authorities since the 2011 uprising and across Libya have failed to protect Libya's Sufi religious sites from attacks and destruction by extremist militias," HRW's Eric Goldstein said.

"The unpunished attacks on Sufi mosques are endangering one of Libya's historic minority communities," added Goldstein, a deputy Middle East and North African director.

HRW said armed groups in Libya "motivated by religious ideology, have attacked and destroyed dozens of Sufi religious sites" since 2011, and kidnapped and killed Sufi followers "with impunity".

Mosques, shrines, tombs and libraries containing ancient scriptures have been destroyed.

The first attack on Sufi sites took place in October 2011 with the destruction of the Al-Masry shrine in Tripoli, said HRW.

The following month armed groups destroyed tombstones at the Girgaresh cemetery in Tripoli and damaged the Sidi Nasr shrine, also in the capital.