File picture: Ahmed Jadallah / Reuters.

CAIRO - Six medical workers were released in Libya after being abducted and held hostage for nearly two weeks by unknown armed men, a leading human rights group said Thursday.

Human Rights Watch's statement said the four physicians, a nurse and an anesthetic technician were released the previous day. Their nationalities were not immediately made available.

Months of fighting between rival Libyan governments for control of the capital, Tripoli, took a further toll on medical workers on Thursday. Two field hospitals near the front lines were hit in separate attacks, killing an ambulance worker and wounding another medical worker in the eye, said Wedad Ben Niran, spokeswoman for the Health Ministry in Tripoli.

The capital is held by an array of militias loosely allied with a UN-supported but weak government. They're fending off a push to take the city by forces loyal to Khalifa Hifter, a veteran army officer aligned with a rival government in the east.

The Health Ministry of the Tripoli-based government released a statement later Thursday denouncing "systemic" attacks on medical facilities by the air force of "aggressors" marching on the capital, a reference to Hifter's forces, saying they only "contribute to exacerbating the daily suffering of the Libyan people."

Hifter's offensive began in April, but fighting has stalled in recent weeks. Both sides are dug in and shelling one another along the city's southern reaches.

Human Rights Watch said the abducted medical workers had been used as "bargaining chips for the release of an imprisoned man," Izzedine al-Wahishi, who's currently being held in the capital.

HRW said he's from the same western city of Zintan where the medical workers' humanitarian convoy was stopped by armed men on October 12.

The statement cited a witness who was in the convoy during abduction and was later involved in negotiating the hostages' release, as well as the medical workers' relatives. They asked to remain anonymous for fear of retaliation.

HRW said the Tripoli-based government "should clarify the legal basis for the continued detention of al-Wahishi held in Tripoli, and either charge him if they have evidence of wrongdoing or release him if they find he has committed no crime."

"The authorities in Zintan should do what they can to hold the captors to account to deter future attacks of this type," the statement added.

Abductions are common in Libya, where the two warring factions rely on armed militias.

Earlier this week, Amnesty International released a report accusing both sides of committing possible war crimes. The London-based human rights group said the sides have launched indiscriminate attacks and used inaccurate explosive weapons in populated urban areas, killing and maiming scores of civilians.

Hifter is backed by neighboring Egypt, the United Arab Emirates, Russia and France. The Tripoli-based government is aided by Turkey and Qatar, in addition to having the UN's blessing.

Oil-rich Libya descended into chaos in 2011, when an international military coalition helped rebels overthrow longtime autocrat Moammar Gadhafi.

AP