Libya's internationally-recognised prime minister said Saturday that military forces in the strife-torn country had united to try to recapture Tripoli and the second city Benghazi from Islamist militias.

"All military forces have been placed under army command to liberate Tripoli and Benghazi soon, inshallah (God willing)," Abdullah al-Thani said in an interview with AFP by telephone from the eastern town of Al-Baida.

Since a 2011 revolution which toppled Libya's longtime leader Moamer Kadhafi, interim authorities have failed to establish a regular army and had to rely on state-backed militias.

Former rebels who fought against Kadhafi have formed powerful militias and seized control of large parts of turmoil-gripped Libya over the past three years.

An operation launched Wednesday against Islamist militias in Benghazi, launched by former general Khalifa Haftar, is "under the command of the regular army and the control of government and parliament," Thani said.

More than 50 people have been killed in the operation.

Haftar launched a first, unsuccessful, campaign against the Islamists in Benghazi in May, but without the support of authorities.

At the time, the authorities accused the former Kadhafi-era general -- who spent years in exile before returning to join the revolution -- of trying to mount a coup.

Before this week's assault, Haftar's forces had been steadily beaten back to a final redoubt at Benghazi's airport, which has come under attack by Islamists since mid-September.

Thani's government and parliament, elected on June 25, have taken refuge in the country's east to escape Fajr Libya, a mainly Islamist coalition which seized control of Tripoli at the end of August.

The fall of the capital followed a weeks-long battle with pro-government militias from the town of Zintan.

Thani said the Zintan forces had also been placed under army command and joined regular units which aim to recapture the capital from Fajr Libya, which he branded as "outlaws" who had set up an "illegitimate" parallel government.