BENGHAZI, Libya (AP) — Clashes between rival Libyan forces for control of Tripoli escalated on Monday as the death toll from days of fighting rose to at least 51, including both combatants and civilians, and the city's only functioning airport said it was hit by an airstrike.

The self-styled Libyan National Army, led by Khalifa Hifter who launched the push on Tripoli, acknowledged striking Mitiga airport, barely 8 kilometers (5 miles) east of the city center.

Hifter's forces have clashed with rival militias that support the U.N.-backed government that controls Tripoli and the western part of the country.

The escalation over the Libyan capital has threatened to plunge the fractured North African nation deeper into chaos and ignite civil war on the scale of the 2011 uprising that toppled and killed longtime dictator Moammar Gadhafi.

The U.N. said the latest fighting has so far displaced 2,800 people. The World Health Organization said two doctors were killed trying to "evacuate wounded patients from conflict areas."

Ibrahim Fadel, an official at Mitiga, said all flights from and to the airport were suspended until further notice following the airstrike. No casualties were reported in the attack.

The official Facebook page of Mitiga, run by the U.N.-backed government, said a fighter jet attacked the facility but gave no other details. A video circulated online shows a fighter jet firing and apparently targeting the airport, formerly a military base.

Maj. Gen. Mohamed al-Manfour of Hifter's Libyan National Army, told the Libyan Address newspaper they bombed targets at Mitiga after receiving information that the U.N.-backed government forces were preparing to target them.

The U.N. humanitarian coordinator in Libya, Maria do Valle Ribeiro, said the clashes around the Libyan capital have prevented emergency services from reaching casualties and civilians, and have damaged electricity lines. The increased violence is also worsening the situation for migrants held in detention centers in the Libyan capital, she warned.

Meanwhile, fighting was underway Monday at Tripoli's former international airport, some 24 kilometers (15 miles) south of Tripoli, which was closed in 2014 after fighting destroyed much of it.

Ahmed Musbah, a resident who lives near the area, said he could hear shooting coming from the direction of the town of Bin Ghashir, to the south. "The sound of fighting seems to be closing in," he said.

Hifter's forces said Saturday they had seized the old airport. However, the militias supporting the U.N.-backed government in Tripoli claimed Monday they recaptured the facility.

The Health Ministry of the Tripoli-based government said at least 27 people, including civilians, were killed and at least 27 wounded since the start of Hifter's offensive against the capital. The media office of Hifter's army said 22 of their troops had been killed since Thursday.

It was not immediately clear when the two doctors whose deaths were reported by the WHO were killed in Tripoli. Ahmed Al Mandhari, WHO's director for the eastern Mediterranean, said that targeting of medical staff was "unacceptable" and "worsens the situation for civilians caught up in conflict."

The European Union's foreign policy chief, Federica Mogherini, called on the warring sides to stop fighting and start talking.

Speaking at the EU's foreign ministers meeting in Luxembourg, Mogherini said all sides in the recent surge in fighting should "go back to the negotiating table under the auspices of the U.N."

Also Monday, U.N. envoy Ghassan Salame said he met with Fayez Sarraj, head of the government in Tripoli, to discuss how the U.N. mission "can assist at this critical and difficult juncture."

Salame later Monday condemned the attack on Mitiga, saying it was "a serious violation of international humanitarian law."

He has asked for "an immediate halt to any further air operations" in order to bring the country back from the brink of what he called "the effective start of a civil war."

Since Gahdafi's ouster, Libya has been governed by rival authorities in the east and in Tripoli, in the west, each backed by various militias and armed groups fighting over resources and territory.

In Cairo, Agila Saleh, head of Libya's east-based parliament, backed Hifter's offensive and the Libya National Army, saying that militias have been "hijacking" the capital.

"The Libyan army moved towards Tripoli with one goal, to free Tripoli from armed militias," he said after meeting with the Arab League's secretary-general, Ahmed Aboul Gheit.

On Sunday, Marine Corps Gen. Thomas Waldhauser, the head of U.S. Africa Command said the United States has temporarily withdrawn some of its forces from Libya due to "security conditions on the ground."

A small contingent of American troops has been in Libya in recent years, helping local forces combat the Islamic State group and al-Qaida militants, as well as protecting diplomatic facilities.

Magdy reported from Cairo.