Men who have volunteered to fight with the military council of the region, which is led by Gen. Khalifa Hifter, are deployed to protect oil ports near the east of the city of Sirte on March 15, 2014. Reuters/Stringer

Two Libyan doctors who worked in field hospitals near the coastal city of Sirte were killed when the Libyan National Army targeted the medical facilities in a series of airstrikes, according to activist messages on Twitter and local media reports . The airstrikes mark the first time that forces loyal to Libya's internationally recognized government launched attacks on medical personnel and humanitarian workers, a crime punishable under international law.

The Libyan National Army, the entity accused of launching airstrikes Sunday on the hospital where the doctors worked, is led by Gen. Khalifa Hifter, a former rogue military leader who has been fighting Islamists in Libya and is now working with Libya's government.

It is not known how many troops Hifter leads, but his forces are said to own heavy weaponry and aircraft that could help the government fight Islamist militias. Hifter has recruited pro-government forces into his army, and Libya's House of Representatives declared a formal alliance with his forces in October. The government body said it would launch an offensive to retake Tripoli, which Islamist militias gained control of in August.

For months, Libya has been embroiled in the deadliest fighting since 2011. Intense clashes are taking place in Misrata, Benghazi, Tripoli and Tobruk, the city where the internationally recognized government is based. An Islamist group known as the Libya Dawn took control of Tripoli in August and set up a rival government, forcing the internationally recognized government to flee.

On Tuesday activists on social media said a bomb-laden car parked in front of the parliament building in Tobruk exploded. Last week in Misrata, government forces launched a series of airstrikes on opposition militias based there.

But the most damaging fighting is taking place near the city of Sirte. The Libyan National Army said Tuesday that it attacked the Gardabiya airbase about nine miles south of Sirte in retaliation for an airstrike launched by Libya Dawn near the port of al-Sidra. That port and the one at Ras Lanuf -- both located just east of Sirte and under control of the internationally recognized government -- are experiencing active clashes. Two oil tanks at the port in al-Sidra are on fire because of clashes, and Reuters reported Tuesday that at least 1.2 million barrels of oil had been destroyed.

The two doctors, both killed Dec. 28, were working in a field hospital about 15 miles away from active ground battle, currently held by Libya Dawm, when the airstrikes landed. The field hospital was targeted three times in just a few days, sources on the ground said.

The United Nations Support Mission in Libya (UNSMIL) issued a statement last week condemning the violence in Libya, and warned that further escalation in hostilities could lead to an "all-out war." The U.S. has not issued a statement on the recent attacks launched by the internationally recognized government, but have issued previous remarks condemning Hifter's interference and violence. The presidents of Egypt and Sudan said in October they would support the Libyan military.