Smoke rises following an air strike as Libyan forces allied with the U.N.-backed government gather after they captured a new area from Islamic State militants in Sirte, Libya, October 16, 2016. (Ismail Zetouny/Reuters)

At least 230 civilians have died in Libya in the chaotic, multiparty air war that has followed the ouster of dictator Moammar Gaddafi in 2011, a report has found.

The study from Airwars, a United Kingdom-based watchdog group, and New America, a U.S.-based think tank, examined the outcome of 2,158 strikes conducted since September 2012 by at least seven local and international actors, underscoring the fraught and fractured nature of post-revolution Libya.

After a NATO-led air campaign helped Libyans topple Gaddafi in 2011, the country has descended into a simmering civil conflict that included the establishment of rival Libyan governments and the rise of militant groups, including a virulent local branch of the Islamic State, that have taken advantage of ungoverned areas to grow strong.

The report, which The Washington Post obtained ahead of its release this week, is the first comprehensive examination of the death toll caused by air operations in Libya’s post-revolution period.

Using social media accounts and other sources to assess individual incidents, researchers concluded that at least 237 and as many as 387 civilians were killed in those strikes. At least another 324 civilians were wounded in those attacks, the report found.

While the civilian death toll appears to be far smaller than that caused by Western air operations against militants in Iraq and Syria, one important feature of the Libyan conflict has been its murky, mysterious nature.

According to Airwars and New America, strikes have been repeatedly conducted in Libya not only by rival local factions and the United States, but also, with far less transparency, France, the United Arab Emirates and Egypt. But fewer than 50 percent of reported strikes have been publicly declared, leading to questions about responsibility and accountability when civilian deaths do occur.

“Not one belligerent has taken responsibility for a single civilian death in Libya since 2012, and that’s a continuation of what we saw with NATO back in 2011,” said Chris Woods, director of Airwars. “It’s a tragedy for the Libyan people.”

Among Libyan actors, the report found that forces led by Khalifa Hifter, who has emerged as eastern Libya’s most influential power broker, have been responsible for the largest share of civilian deaths since 2012. Backed by outside powers including Russia, Hifter has vowed to clear Libya of extremist militants who have taken root since Gaddafi’s death.

But Hifter has been criticized for harsh tactics and his refusal to support the Western-backed Government of National Accord (GNA) that seeks, in a rivalry to his own power, to assert authority across Libya.

Hifter’s self-proclaimed Libyan National Army is believed to have conducted about 1,100 strikes resulting in at least 95 civilian deaths, and potentially as many as 247, researchers found. Most of the attacks by Hifter’s forces have taken place in the eastern cities of Benghazi and Derna and in the country’s central oil region.

The GNA, which has fewer air assets, has conducted about 54 strikes, probably resulting in seven to 63 deaths, the researchers said.

Of the outside powers involved in post-revolution Libya, the United States has played the most significant military role. According to Airwars and New America, U.S. aircraft have conducted about 525 strikes on militants in Libya since September 2012.

That included an intense 4 1 / 2 -month operation in 2016 to push the Islamic State out of Sirte, the coastal city that became the group’s most important stronghold outside of Iraq and Syria.

U.S. Africa Command has said its operations have caused no confirmed civilian casualties in Libya during that period, but Airwars and New America assessed they probably resulted in 11 to 75 noncombatant deaths.

U.S. estimates of civilian casualties have been far lower than those from outside groups, partly because the U.S. military does limited follow-up after strikes to assess whether civilian deaths took place.

An AFRICOM spokesman, Col. Mark Cheadle, said the command had not seen any credible allegations of civilian deaths linked to U.S. operations in Libya. “We have not seen any further evidence or information which would change our conclusions,” he said.

The difficulty the United States faces in Libya, where it relies on electronic and aerial surveillance, was visible in the disputed aftermath of a 2015 strike targeting Mokhtar Belmokhtar, a legendary Algerian militant, and a 2016 attack that the Serbian government said killed two kidnapped diplomats.

In both instances, the facts of what occurred during those attacks remain disputed.

The researchers said the United States has accounted for its strikes in Libya with greater, though not complete, transparency compared with other countries that have sent planes into Libya.

Often, it has been difficult to determine who has been behind a particular strike in Libya, with no local factions or foreign actors taking responsibility and locals assigning blame to multiple parties.

The UAE, which has been eager to ensure that Islamists do not seize control of Libya’s weakened state institutions, is believed to have conducted at least 35 strikes in Libya during that period. Egypt, which shares those concerns and has seen attacks on a number of Egyptians within Libya, is believed to have conducted at least 41 strikes.

“This is one of the world’s forgotten wars,” said Peter Bergen, a vice president of New America. “The NATO intervention didn’t end the conflict; it precipitated a new phase and Libya is now an arena for proxy warfare.”

Researchers say the smaller civilian death toll in Libya — compared with over 6,000 in Iraq and Syria — is due to Libya’s lower population density and, perhaps, poorer reporting. Mobile phone towers have been targeted in Libya’s conflict, and the country lacks the kind of watchdog groups that have played an important role in the Syrian conflict.

On Wednesday, AFRICOM said it had reviewed all “available relevant information” concerning allegations of civilian deaths in a June 6 airstrike in central Libya and determined that no unintended casualties occurred. While AFRICOM said that four Islamic State militants died in the strike, Airwars said there were indications that appear credible that three civilians were killed.