More details have emerged from the massive battle in Syria that is said to have killed hundreds of Russian contractors — and it looks as if they tried to test the US.

Reports have said that the forces loyal to the Syrian government advanced and fired at a US-held position in Syria and that the counterattack obliterated them.

It looks as if the Russian contractors and their Syrian and Iranian allies may be too weak to budge the US without getting the Russian military involved.

More details have emerged from a massive battle in Syria that is said to have pitted hundreds of Russian military contractors and forces loyal to the Syrian government against the US and its Syrian rebel allies — and it looks as if it was a mission to test the US's resolve.

Bloomberg first reported this week that Russian military contractors took part in what the US called an "unprovoked attack" on a well-known headquarters of the Syrian Democratic Forces, a rebel cohort the US has trained, equipped, and fought alongside for years.

Reuters cited several sources on Friday as confirming that Russian contractors were among the attackers and that they took heavy losses. The purpose of the attack, which saw 500 or so pro-government fighters get close to the US-backed position in Syria, was to test the US's response, Reuters' sources said.

How the battle played out

US forces fire off artillery rounds in Syria. US Marine Corps

Initial reports said pro-government forces launched a coordinated attack that included about 500 troops, 122mm howitzers, tanks, and multiple launch rocket systems.

A source close to Wagner, the Russian military contracting firm, told Reuters that most of the troops were Russian contractors and that they advanced into a zone designated as neutral under a deal between the Russian military and the US-led coalition against the terrorist group ISIS.

The troops reportedly sought to find out how the US would react to the encroachment into that zone.

Forces operating Russian-made T-55 and T-72 tanks fired 20 to 30 tank rounds within 500 feet of the SDF base, which held some US troops, said Dana White, the Pentagon press secretary, according to the executive editor of Defense One.

The US-led coalition responded with "AC-130 gunships, F-15s, F-22s, Army Apache helicopter gunships, and Marine Corps artillery," according to Lucas Tomlinson, a Fox News reporter. CNN also reported that Himars and MQ-9 drones were used in the attack.

"First of all, the bombers attacked, and then they cleaned up using Apaches," attack helicopters, Yevgeny Shabayev, a Cossack paramilitary leader with ties to Russia's military contractors, told Reuters.

The Reuters report cites an unnamed source as describing Bloomberg's report that 300 Russians died as "broadly correct."

The US reported more than 100 dead. According to Reuters, Russia says only five of its citizens may have died in the attack.

The Pentagon says only one SDF fighter was injured in the attack.

What might the Russians have learned from the 'test'?

Russia's military aircraft at a base in Syria. Ministry of Defence of the Russian Federation

The pro-government forces operated without air cover from Russia's military. The US-led coalition apparently warned Russia of the attack, but it's unclear whether Russia's military passed on notice to the troops on the ground.

"The warning was 20 minutes beforehand," a source told Reuters. "In that time, it was not feasible to turn the column around."

Reports have increasingly indicated that Russia has used military contractors as a means of concealing its combat losses as it looks to bolster Syrian President Bashar Assad's flagging forces. Russia has denied it has a large ground presence in Syria and has sought to distance itself from those it describes as independent contractors.

According to the news website UAWire, Igor Girkin, the former defense minister of the self-described Donetsk People's Republic, a separatist region backed by Russia in eastern Ukraine, said last week that Russian mercenaries operating in Syria who died in combat were cremated on sight to hide the true cost of Russia's involvement.

As the US's stated mission in Syria of fighting ISIS nears completion, others have taken center stage. The US recently said it would seek to stop Iran from gaining control of a land bridge to Lebanon, its ally, citing concerns that Tehran would arm anti-US and anti-Israeli Hezbollah militants if given the chance.

The US also appears intent on staying on top of Assad's oilfields in the east both to deny him the economic infrastructure to regain control of the country and to force UN-sanctioned elections.