AL AZIZIYAH, Libya — At first, the fighters inside the empty school building, about 20 burly, heavily armed Russian mercenaries in dark fatigues, seemed trapped, recalled some of the Libyan militiamen who were there.

But as the Libyans pushed forward, snipers opened fire from inside with high-powered rifles. Within minutes, three Libyan militiamen were killed, all shot in the head.

Hundreds of Russian mercenaries, many highly trained and well armed, are fighting alongside renegade Libyan commander Khalifa Hifter as he seeks to oust the country's United Nations-backed government, according to Libyan military commanders and fighters, as well as U.S. military and other Western officials.

These foreigners fighting for Hifter's self-described Libyan National Army are introducing new tactics and firepower on the battlefield, threatening to prolong the most violent conflict in this North African country since the Arab Spring revolution eight years ago.

"The entry of the Russian forces into the war has altered the battlefield," said Osama al-Juwaili, a top commander of the Libyan government's forces. "Their presence complicates things for us."

They represent the latest escalation in Libya's proxy war, which has drawn in European and Arab countries — notably the United Arab Emirates and Egypt — despite an international arms embargo. And the arrival of these mercenaries comes at a time when Russia has been expanding its military and diplomatic reach across the Middle East, Africa and beyond, enjoying greater clout in places such as Syria where the United States is disengaging.

"We are aware of Russian private military companies operating in Libyan National Army-controlled territory in eastern Libya, and they have also operated in western Libya," said Rebecca Farmer, a spokeswoman for the U.S. military's Africa Command.

Farmer said the Russian mercenaries work for the Wagner Group, a private army that experts have linked to Yevgeniy Prigozhin, a close ally of Russian President Vladimir Putin. The Wagner Group has previously appeared in combat in Syria, the Central African Republic, Ukraine and other countries considered strategic for the Kremlin's geopolitical and economic interests.

Russia has arms and construction agreements worth in excess of $4 billion, made with late Libyan dictator Moammar Gaddafi, who was ousted and killed in the country's 2011 uprisings and NATO intervention.

"They have a strong economic rationale in their continued support to Hifter," said Farmer, referring to Moscow.

A senior Western official described the Russians as "guns for hire" and said, based on the analysis of intelligence and military experts, that these mercenaries were believed until recently to number around 300. But new, "very alarming" information indicates there are thousands, said the official, who spoke on the condition of anonymity because of the sensitivity of the issue.

Once largely based in Hifter's eastern Libyan strongholds, they are "nowbeing brought to the front lines," said the official, adding that these fighters include snipers and artillery experts and have brought "some tactical skills and edge to the fight."

Senior military commanders for the Libyan government have estimated the number of Russian mercenaries at around 300 based on their intelligence sources inside Hifter's territory. "We have eyes on the ground there," one top commander said.

Some pro-government fighters said they knew the mercenaries were Russian by the chatter on their hand-held radios. The two sides can access each other's frequencies at times, and the pro-government fighters recall hearing Russian spoken. In the battle for the empty school, fighters said, they heard the enemy fighters screaming commands and names in Russian.

A Washington Post reporter also reviewed Russian identity cards, documents and other material belonging to the Russians found at the site of clashes, as well as photos and videos of the mercenaries taken by Libyan militia fighters.

A spokesman for Hifter's Libyan National Army, Col. Ahmed al-Mismari, said the reports of Russians are "fake news" and that "all our fighters are Libyan."

Putin's spokesman, Dmitry Peskov, declined to answer questions sent by the Post about the mercenaries, replying that "the Kremlin does not have this information," while a spokesperson for Prigozhin said the businessman "has nothing to do with the socalled 'Wagner' private military company" and declined to comment further.

The Russian mercenaries entered Libya in September, according to Libyan commanders and fighters, six months after Hifter launched a surprise offensive on the capital. The 75-year-old commander, a former general in Gaddafi's army who is a dual U.S.-Libyan citizen and lived for years in northern Virginia, is aligned with a rival eastern government.

His forces, composed of eastern militias, are battling armed groups from Tripoli and other western cities aligned with the Government of National Accord. The conflict has killed more than 1,000, including at least 100 civilians, and driven more than 120,000 from their homes, according to the World Health Organization.

In addition to the United Arab Emirates and Egypt, Saudi Arabia is backing Hifter, as is France. Italy and other European nations, as well as Turkey and Qatar, are supporting the Tripoli-based government.

U.S. policy has been uncertain since April when President Donald Trump endorsed Hifter's offensive in a telephone call.

The former Soviet Union had a close relationship with Gaddafi, sending weaponry and military advisers to Libya throughout the 1970s and 1980s.

In Hifter, Moscow sees an opening to gain back billions in lucrative oil and military contracts that it lost when Gaddafi was killed, analysts said. Russia has printed billions of Libyan dinars to prop up eastern Libya's economy and help finance Hifter's military campaign. Russia has also blocked a U.N. Security Council statement that sought to condemn Hifter's offensive.

Along the front lines in southern Tripoli, Libyan fighters ran fast between houses pocked by mortar shells and bullets to avoid being caught in the sights of Russian snipers.

On the second floor of a half-destroyed mansion, commander Khalifa al-Naluti peered out at an apartment complex 100 yards away. "The Russians are over there," he said. "It's dangerous to stand here."

Libyan fighters on both sides are undisciplined, often firing excessively and haphazardly at targets. The Russians, by contrast, move in small groups and attack from side positions, mostly at night or in the early morning hours, Libyan fighters say. The Russians preserve their ammunition, firing at optimal moments with precision.

"Their fighting style is different than what we are used to," said Jabber Abu Dabous, a militiaman. "They fight in a professional manner."

And the Russians may not only be fighting but also training Hifter's forces. Since the arrival of the Russians, Hifter's forces have begun using novel military tactics and new weaponry, say pro-government militia fighters.

At the only field hospital in alAziziyah, a small town 25 miles southwest of Tripoli, doctors have been treating new types of war injuries over the past several weeks.

Nearly every bullet wound is now in the chest or head, reflecting the snipers' expertise, said a senior surgeon who spoke on the condition of anonymity because he feared for his relatives who live in Hifter-controlled areas. Mortars, too, have been striking targets with greater precision. "They've become more accurate in the last three weeks," said the doctor.

In at least six deaths, the bullet entry wounds were unusually small. The bullets did not exit the body, suggesting the presence of modern guns and ammunition, he said. In previous conflicts, the wounded arrived during the day, as the fighting usually ended by dusk. Now, they are seeing more casualties arriving in the pre-dawn darkness.

"The timing of battles, the types of injuries, the way people are dying, it's all changed now," said the doctor.

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