Destroyed buildings in Libya's eastern coastal city of Benghazi on Oct. 20, as clashes have continued to rage in the area. (Abdullah Doma/AFP/Getty Images)

For months, United Nations negotiators have been racing to settle a feud between competing governments in Libya, a rivalry that has crippled the oil industry, provided a foothold for the Islamic State and plunged the country into civil war.

On Tuesday, after the mandate of one of two rival parliaments — the only one recognized by Western powers — lapsed before lawmakers could endorse a proposed unity government, both of Libya’s dueling governments were left without clear legal standing and international backing.

The failure of warring parties to support a U.N. peace proposal threatens a U.S-backed effort to stabilize Libya and pushes the country into uncharted territory.

“As of today, neither of those governments can claim legal legitimacy — making Libya the largest piece of terra nullius, or vacuum of sovereignty, on earth,” said Jason Pack, a researcher of Libyan history at Cambridge University and president of Libya-Analysis.com.

The latest turn in Libya’s fraught post-revolution path comes exactly four years after rebels hunted down and killed former dictator Moammar Gaddafi on Oct. 20, 2011, marking a bloody end to the strongman’s long rule and punctuating a NATO air campaign that was seen, at the time, as a model intervention in the Middle East.

Four years later, Libya is broke, isolated and plagued by insecurity. For more than a year, the country has had two prime ministers, one in the western city of Tripoli, the other near the border with Egypt in the east. There are two self-proclaimed parliaments, two central banks and two national oil companies. The authority in the east was the one recognized by the United States and its allies.

Across Libya, well-armed militias retain massive power, sometimes turning their fire on each other. The de facto partition has made it harder to contain the expansion of militants with the Islamic State, who have set up the group’s strongest affiliate yet outside of Iraq and Syria.

Libya’s downward spiral has accelerated just before presidential candidate and former U.S. secretary of state Hillary Rodham Clinton — who has described the 2011 Libya intervention as “smart power at its best” — is set to appear at a politically charged hearing about the 2012 attacks that killed four Americans in Benghazi, Libya.

Politicians from the western and eastern Libyan legislatures continue to feud over the proposal for an interim unity government by U.N. envoy Bernardino León.

If approved by both sides, León’s plan would extend the mandate of the eastern parliament, known as the House of Representatives, and create a new state council that is expected to be dominated by figures from the western legislature, the General National Congress.

This week, foreign ministers from 12 countries and the European Union urged Libyans to embrace León’s proposal, saying the country faced a “stark choice” between stability and further chaos. Western nations have said they will not resume military or civilian aid to Libya until there is a single government.

Earlier this month, the Libyan House of Representatives voted to prolong its mandate beyond Oct. 20. So far, Western governments do not appear to back that decision as a valid extension of the chamber’s legislative authority.

“This action is not a substitute for completing the political agreement to form a government of national accord, which is essential to help Libya unify and move forward to fulfill the needs and aspirations of the Libyan people,” said Jonathan Winer, the State Department’s special envoy for Libya.

While lawmakers in eastern Libya appeared to reject the U.N. plan on Monday, U.S. and U.N. officials hope that debate will continue in coming days.

In the west, the General National Congress is similarly divided over the U.N. plan. In addition to the makeup of the hoped-for interim government, Libyan politicians are most sharply at odds over the future of Khalifa Hifter, a prominent general who has won support in eastern Libya but remains vilified in the west.

“The declining legitimacy of the existing Libyan institutions is all the more reason . . . why Libya needs to move forward on a government of national accord that can govern the country,” said a senior U.S. official, speaking on the condition of anonymity to discuss the situation freely.

Abdurrahman Sewehli, a high-profile member of both parliaments, said that majorities from both sides may be willing to eventually support the agreement. “The time is not late for putting the train back on the tracks and finish[ing] this particular journey,” he said.

León has been criticized by both eastern and western factions for favoring the other side.

In practice, it’s unclear what a lapse of the sole widely recognized authority in Libya may mean. The eastern government already controls far fewer monetary resources than officials in Tripoli, Libya’s longtime capital. This week’s developments could make lawmakers in Tripoli less likely to compromise with their eastern counterparts or increase the eastern government’s reliance on outside powers such as Egypt.

But Mattia Toaldo, a Libya expert at the European Council on Foreign Relations, said mounting doubts about the legitimacy of both governments would further impede operations by international oil firms, which are needed for the continued payment of government salaries, and could keep billions of dollars in Libyan assets tied up in overseas legal disputes .

“It does create a big problem for the West, because what if none of the two governments takes a decision in the next few weeks — who do [the Western governments] deal with?” Toaldo asked.